Let me just start by saying that the Canon 17-55 is just not an option. If I was going *that* expensive, I'd just hold off for the 24-70L or 16-35L ... which is, perhaps, what I should do anyway.
I've looked at the photozone.de reports, and they seem a lot less conclusive to me. While resolution on the Tamron shows some improvement, they appear to be using a different metric. Furthermore, only the Canon has results for the 15MP sensor; so i don't have a good idea what the Tammy is capable of on the T1i. That said, the results for the Tammy in the tests are impressive ... then again, I have a hard time reconciling those results with the samples here on POTN.
As helpful as all y'all are being, I don't know that I'm any closer to a decision at this point. LoL! Two key questions that some of y'all have raised definitely need a better answer.
1. Am I going full frame? Given the results that come out of the 5D and the 1D (still a crop, I know), I suspect I will make that move some day. When that day will come is a mystery to me. Could happen sooner (with the help of grant funding, since this is part of my research) ... and it could happen later (by me squirreling away $$ in a jar month-to-month). So I'm confident it will happen someday; but I really don't know when. In the next year? Probably not. In the next 2-3 years? Possibly.
2. What are my lighting conditions? I don't purposefully go out at night to take photos. One of my favorite pastimes is to walk around a city during the day and capture what I see. That said, the vast majority of my photos at this point are probably candids of friends and family in mixed light situations indoors (think living rooms with incandescents and bars). Up until christmas, I was using either no flash (with a high ISO) or the on-board flash. Once I got my 430EX II, I discovered the joys of the bounce flash, which my friends and family don't seem to mind (any more than they mind my paparazzi butt clicking away without a flash, that is). My subjects are usually adults, but friends and family have children who I try to catch "in the act" before they notice me an smile awkwardly.
All that said, I appreciate the comments and feedback and hope y'all keep it coming. I'm still (obviously) trying to think this out and make a decision ... which is a lot harder that I was expecting.