It depends on what you mean by utopia, and also how we define "middle class".

To start with the latter, there was no middle class in the Athenian consciousness; there was a philosophical ideal of a middle between two extremes, but in practice the citizenry was ideologically divided between "the rich" and "the poor". The cutoff point was whether one had to work for a living, or whether one had the wealth (and slaves) to live off the work of others. Everyone who had to survive on his own toil was considered "poor". If we want to find an Athenian middle class, we will have to impose some modern standards on the sources. I suspect what you're looking for is not really a member of this artificial "middle class" (a term that comes with a lot of modern ideological baggage), but simply an Athenian citizen who is not desperately poor but also not one of "the rich".

The problem then becomes that this vague socio-economic class would have included a very wide range of people. The territory and population of Classical Athens was comparable in size to that of modern Luxembourg. Life for a skilled worker living in the crowded, urban, ethnically mixed Peiraieus would have been very different from that of a small farmer herding goats in the mountains on the border of Boiotia, three days' travel from Athens. The men who worked the land around Acharnai, the second largest settlement in Attika, would have had a different outlook on life from those who gambled their fortunes on trade with the Phoenicians and Etruscans. How do we define what their life was like in general?

It's worth pointing out that relatively few of the men in this arbitrary middle would have actually lived in Athens itself. Those who did not have enough wealth to have slaves do all the work for them would not have the spare time to hang around at Athens, hours or even days away from their fields and workshops. The city, then, was the playground of absentee landlords, who engaged in politics and the pursuits of leisured life, and of the so-called "naval mob", the urban poor who relied on the wages of the Athenian democracy for their livelihood. The Pnyx, the hill on which the Assembly was held, had a capacity of about 6,000; apart from the men in the Council, who were drawn from across Attika, most of the men who voted would have been wealthy political high-rollers and poor men eager to receive the compensation (1 drachma, a good day's wage) paid out to those who attended.

For the rest of the citizen body, the great democracy of Athens must have seemed like something that happened far away, unless they were specifically called upon to contribute. This could happen in a number of ways. Every year, a specific number of men were selected by lot from the citizen rolls to make up that year's jury courts - a nice way to make an extra buck, but also a way to exert direct influence over the laws of the state. Every year, too, the 500 members of the Council were selected by lot from eligible citizens, with a set number of Councillors provided by each of the demes into which Attika was divided. The Council ran the day-to-day affairs of the city and set the agenda for the Assembly, giving them considerable power to determine the course of the state. Finally, the citizen body was eligible for military service; many of the people on the high end of the "middle" would have been on the List, which meant they could be called up several times a year in wartime, and might be called up to fight overseas for years at a time.

All of these aspects of the democracy could be regarded both as an unrivaled opportunity to exercise personal power and influence (setting policy, upholding the law, defending the community) and as a terrible burden on the already difficult life of a subsistence farmer or skilled worker. The fact that the Athenian state eventually instituted pay for all the services it required (jury, Council, Assembly, and military service) was certainly one of its most democratic features, ensuring that no citizen would be forced to default on his public duties. Even so, it is easy to see why many Greek populations were happy to leave the business of government and war to the leisured elite.

In what other ways was life in Athens different? All Greek states had a calendar of public festivals, funded by the state or by generous contributions from wealthy citizens. Athens, however, famously had one of the most extensive such calendars, and one 4th-century orator complained that the city spent more on festivals than it did on its many wars. Great religious celebrations such as the Panathenaia or the Dionysia involved processions, theatre performances, dance competitions, sacrifices and public feasts - all of which both Athenians and foreign visitors could take part in. Like all Athenian public activities, the cost was largely shouldered by the elite, who were assigned the duty of funding parts of each festival as a responsibility to their community. The rich paid; everybody else enjoyed the results. These results - theatre plays in particular - remain among the most treasured and lasting products of Classical Greek culture.

However, it is not clear to what extent the so-called middle was involved in any way other than as spectators. Tragedies tended to be about mythical kings and their trials and tribulations; they highlighted the lifestyle and works of the leisure class, often (as in Aischylos' description of the battle of Salamis in his Persians) at the expense of the achievements of ordinary people. Comedies may have reflected the common man more closely, but even there, the focus is often on prominent politicians or public figures; Aristophanes also liked to poke fun at the lifestyle of the very wealthy, whose sons served in the cavalry. While the Athenians stereotyped themselves as hoplites in wartime, rather than the more elite cavalry, even hoplites clearly still represented the upper stratum of society, unattainable by the majority of citizens. In many ways, the way the Athenians represented themselves in public (both politically and militarily) could be compared to the modern ideological "middle class", in the sense that few actually live the reality of it, and many more aspire to it and would like others to believe they are part of it.

This has ended up being a bit of a rambling post... Sorry about that - it would help if you could define the ways in which you've seen Athens described as a utopia, so I could focus my answer on the extent to which that is justified. Also, feel free to ask if anything is unclear.