I'm going to suggest the 50D as well, not just because I own it, but because Canon has decided to cheapen the once great X0D line. The 60D isn't a replacement for the 50D, rather it's a stripped down version that has a higher mp sensor, but the body is smaller, plastic and missing a lot of the features of the X0D line. The joy stick is gone, the top buttons are single, rather than dual function, AF Microadjust is gone, and it has an articulating screen. In short, it's been dumbed down to appeal to "enthusiasts" rather than serious hobbyists. However, it has video and wireless flash control, missing on the 50D. As far as the T3i, the previews I've read seem to show there's not much different from the T2i. Optically it's the same, but has some of the features of the 60D, so basically it's a T2i on steroids. As far as that goes, there doesn't seem to be much difference between the T3i and 60D, except size, weight and ergonomics. IQ is predicted to be the same between the T3i and 60D. High ISO performance on either of these is better than the 50D, and that should be noted, but it's not really a deal breaker in my book. I have NR software.
At this point, if I were to upgrade, the only direction for me would be to move up to the 7D. Seems that's the only prosumer model Canon makes now. You, however, aren't me. So if video is important to you, obviously a 50D won't work. If the features missing from the 60D/T3i are important, than the 50D wins. The 60D is a nice camera, but for someone who already owns an X0D body, it's a step down. Going from an older Rebel, it's still a step up, but in terms of a Rebel TXi, not so much.