In Origins of the Korean War (1990), Bruce Cummings argued that the Korean War was local and the basis for the belief of Soviet involvement was US cold war propoganda. Really bad timing for Dr. Cummings. In 1991 Russian authorities started declassifying the Soviet archives, which revealed that Kim Il Sung had sent dozens of telegrams begging Stalin for authority to invade, and that the two met in Moscow repeatedly to plan the event.

It is generally accepted, to my knowledge, that there were Soviet advisors in Korea (not unlike US advisors in Vietnam later), and there were indeed Soviet pilots and materiel there. The Soviets took US weapons back to study them, too. For that matter, Stalin had an effective veto power over peace, and it is not an accident that a cease fire began in July, after Stalin died in March of the same year (1953). At this point China was not nearly as independent of Moscow as it would soon be, but Kim Il Sung was largely reliant on Stalin.

See this article by Katheryn Weathersby for much more very interesting reading.

edit: clarity