The 50D's main advantage over the T1i is its much faster shooting speed - 6.3 frames/second is one frame per second faster than even the 60D and almost double that of the T1i. Its 9 AF sensors are all cross-type, as opposed to the T1i whose only cross-type sensor is the middle one. But at the price the 50D sells for ($1000), you'd generally be much better off with the 60D which betters it in other respects, many of which are probably more important: 19 cross-type sensors versus the 50D's 9 is certainly a numeric improvement.
The 60D or 7D seems like a good upgrade - I'd be hesitant to try to shoot any pro gig with the T1i but it's certainly been done. The interesting feature the 60D provides over the 7D is the tilting screen, which you could use to take shots over your head (or at ground level without lying flat) with Live View. The 7D makes this harder to accomplish.
Personally, working strictly as an amateur, I think it's a bit early to upgrade the T1i simply because I still get perfectly good results from it. There's essentially no resolution bump to be had from the 15 to 18 megapixel bump; the ISOs seem improved but I think it's better to skip a sensor (and possibly DIGIC processor) generation entirely. With any luck we're not too far off from a new generation of cameras, and at the very least there may well be a T4i coming out on schedule next year - I don't see it being yet another 18 megapixel camera but I'd still wait and see.
When you say "concert gigs," do you mean in low light or outdoors? I would think that clean higher ISOs (above 800) and autofocus accuracy are your biggest concerns there - that might prompt you to go for something better than the T1i ahead of schedule. If possible I would try to shoot some test scenes and see how it works out just to get prepared.
