Wow, what great comments; this is good stuff!
Anyhow, here are my thoughts. I would upgrade to a full-frame camera if I thought it would benefit me proportionally to the cost. The 5D is twice as expensive than the 40D, and I hear that it is not weatherproofed/not as solidly constructed as the 40D (honestly, this is a much bigger concern for me). Besides, I kind of like the fact that my EF zoom lenses get an extra kick from the 1.6 crop factor! Also, my favorite lens is an EF-S lens, so unless anyone here can REALLY convince me that going to full frame is worth it with my budget, I'm not going to ditch it.
I'm still going to wait and see what comes at photokina. Hell, I might see the introduction of the 50D and decide to buy a new 40D at a steal of a price! I'm pretty happy with my technique, and with my current camera. I have, however, held and used a friend's 10D, and boy, was there a difference between that and the XT! It was a bit heavier, of course, but it felt solid as a rock, and being able to change aperture and shutter speed without holding down a button while turning a wheel seems was AMAZING. I know, it seems like a small thing, but it's a big deal to me. Plus; the IQ is supposed to be far better than the XT. I know, they have the same sized sensors, but size isn't everything! They have different types of sensors and different image processors, so it's likely that the 40D has superior IQ.
I'm no professional, nor do I plan to be one in the future. But as a field biologist and general lover of nature, I plan to be outside taking pictures VERY often. I am in need of a camera that will serve me faithfully in all climates and conditions of weather for a few years. As far as I can tell, the 40D is the only currently existing model that will provide this type of service while remaining within my budget (~$1,000).