the way to look at it would be like this...
the Resolution on the Ipad 3 is no less than about double if not more than the Ipad2, so will apps for the Ipad 3 work on an ipad2? the short answer would be MAYBE...
The reason being, if these apps are designed to use up the ipad3's specs, the ipad2 may not have the processing power needed to handle the app...
But with that being said, not sure how many developers are really focusing on creating apps for the ipad 3 only, so i would guess if you went with an ipad 2 you would still be good for at least a year if not more before you really would have to worry about whether or not a lot of the apps will work on the ipad2
Now as for those who say get Android, you also have to consider, that a lot of developers who develop apps for certain models of android platforms, since Android systems on different devices run slightly differently, which means not all apps are useable across all android mobile platforms... which is why i personally prefer apples platforms... since besides the fact they look a lot better, they just tend to last a lot longer than any other brands products imho...