The exterior dimensions for the iPad 4G are the same at 241.2 mm by 185.7 mm, the only dimension that differs is the depth. The depth of the iPad 4G was increased from 8.8mm to 9.4mm or 0.023 inches. It is safe to bet that many iPad cases weren't built with this tight of tolerances, with the exception of the snap-on hard cases. I really doubt that the iPad 4G weight increase or distribution will affect iPad case functionality.
iPad 2 cases working with iPad 4G:
- iPad 2 SmartCovers
- Most Keyboard Folios
- Fabric Bezel lining
- DODOcase (seems to sit deep enough)
iPad 2 cases not working on iPad 4G:
- HardShell From HardCandy
- Powis iCase
- Any case that the iPad 2 "snaps" into
DISCLAIMER: I am not a 3rd generation iPad caseologist, nor do I have an retina iPad (yet). These are basic assumptions made, but risk always exists in applying a accessory to a product it was not designed for...we are not responsible should your new iPad fall out and land on your big toe.