Canon just announced the Rebel T4i, which is a slightly improved T3i.
For the T4i, Canon kept the same sensor but included their improved software and processor. The camera has a (supposedly) improved auto-focus system.
The T4i can continuously auto-focus while recording video--and silently with Canon's new lenses that they also just announced. This camera can record sound in stereo. These are good advancements, but are more-or-less gee whiz if you are not using the camera much for video. If you are looking for a DSLR that is somewhat inexpensive and has good video capabilities, this may be the camera for you.
The main improvements Canon gave the T4i over the T3i are in video capabilities. The improvements in image quality are very minor and are probably not noticeable even with a close side-by-side study.
What is disappointing is that the T3i was not rated very highly by DxOMark, and Canon did very little to improve that with the T4i. Nikon, on the other hand, just released the D3200, which ranks much, much higher at DxOMark, yet retails (with a kit lens) for $250 less.
If you are not a Canon owner and video isn't really important to you, The Nikon D3200 is both significantly better and significantly cheaper--it's a no-brainer. If you are a Canon owner, it doesn't make much sense to change brands since you've already spent money on lenses and such. The T4i is not improved enough to justify the replacement of your T3i, and probably not your T2i, either.
The only reason to buy the T4i is for it's video capabilities. But that also means buying Canon's new "silent" lenses. You'll have to decide if it's worth it to you.