At the Constitutional convention, one of the Pinckneys present offered a full blown version of the "slavery is a positive good" argument.  Similarly, you will find the same argument presented in the numerous Virginia county petitions offered in opposition to the relatively small number of Baptist and Methodist petitions in the early 1780s to abolish slavery in Virginia.

John Kaminski, who is as authoritative a scholar of the Founding as anyone on this list, concludes in his excellent documentary collection on Slavery and the Founding that the opportunity to abolish slavery, afforded by the liberal rhetoric of the Revolution, had pretty much closed down by 1787.

I think it is a bit misleading to suggest that the founders, as a group, were overtly critical of slavery.  Some were--but a good many were not.  Certainly the majority of enfranchised southerners had no difficulty at all reconciling slavery with their commitment to the Revolution.  That is true of a good many of their northern fellow-citizens.
Kevin R. Hardwick, Ph.D.
Department of History
James Madison University