From my experience (and I have all the iPads!) all my iPad2 cases fit the iPad3 (I have 3).
The 3 is 0.8mm thicker. In theory a case made specifically for the iPad3 should support that thickness by design, but I have so far found that all the iPad2 cases are OK. The only exceptions seem to be the very tight fitting rigid plastic cases which folks have said can be a little "tight" around the corners on a 3 (due to the extra thickness).
The other problem which has been reported are on cases which use the "smartcover magnet" feature to turn the device on and off automatically. Apple have changed the polarity of the magnet in the iPad3. This means that on old cases, it is a 50/50 chance whether the magnet was inserted the right way round to suit the new iPad. I am lucky. I have an old smartcover which works on my 3. But folks have reported that lots of old cases don't have the magnets the right way. Apple have obviously adjusted their manufacturing process (a while ago) because any recently manufactured smartcovers work perfectly on both devices.
In summary, you should be OK, unless its an old smartcover in which case its a 50/50 chance!
Hope that helps!.
