Dustin, you were right. A closer examination of the chain (brand new shimano) showed a pin starting to work it's way out. The LBS couldn't find a new Shimano pin, I didn't think about just bringing the bike home and pressing the pin back myself, and really, I wanted a chain with a removable link. So I bought an SRAM chain and now the derailleur just purrs.
Thanks for all the help.