The above image is from my previous Lovebird post and was processed with Photoshop CS4. As you can see, it was fairly noisy as it was shot in low light at ISO 3200 (with a 5D). When using CS5 to open a photo previously processed in CS4, all the previous settings as well as the CS4 processing is used. A small icon appears (denoted by the red circle above) which lets you "update to current process."
As you can see the new CS5 processing is of incredibly superior quality. The new noise reduction and sharpening controls would let me further improve the image quality beyond the inherent processing improvements seen above. Now I feel like I have to re-process every low light photo I've ever taken.