I've read that for all their lamentations over the evils of slavery, only George Washington among the southern Founding Fathers bothered to free his slaves in his will. Was this true? How unusual an act would this have been for the time?

There was a wave of manumission in Virginia following the American Revolution and some 10,000 slaves were manumitted in the 1790's. This was part of an increased dialogue that had centered on the evils of slavery (in particular the slave trade) in both the United States and Great Britain as well as a loosening of the manumission laws in Virginia.10,000 is a very large figure in American history for manumission although comparatively small to Latin American slavery. So Washington wouldn't have been particularly out of place for manumitting his slaves but given the number of slaves he owned certainly set him apart(although most he couldn't free as they were from his Wife's first husband). Regarding other Southern founding fathers, the only prominent Virginian of the period that comes to mind is John Randolph of Roanoke. A rather interesting and eccentric figure even for his time period, he left competing wills behind that were contested in court by his heirs regarding the manumission of his 400 or so slaves that dragged on for over a decade. Even after they were freed they had a horrible time actually making it to the land they had bought in Ohio being met with angry armed mobs repeatedly who did not want to live among blacks. Randolph really didn't come into prominent political office until the 1790's however so past the time you'd probably normally consider the founding.