Thanks for all the ideas you've given me! I'm going to check under the bb at the cable guide -- though it seems unlikely to be the issue as the cables are new and I can't eye ball any issues from friction coming from that area. I also use full-coverage fenders with huge, pavement-dragging flaps so I don't tend to get a lot of debris under there.
dddd: I see what you're talking about. The shifter does indeed spring back into the...well, I guess the correct position, what I meant is that I can physically grab the lever and pull it slightly to get some cable movement. I don't think it tends to move by itself, but I could be wrong. I guess this little bit of movement is something designed into the shifters to facilitate down shifting - like you mentioned. I didn't know that.
I'm going to try it out, and make very minor adjustments to cable tension and see how it goes. I'll update after this week of commuting.
Thanks,