I recently found a quick workaround for a problem I had quite often, maybe it can help somebody else:
In our project setup we have lots of jars without debug info, but we have the library source code (we should instead strip out the debug info right before deployment, but I can't change that).
Now if a crash occurs in one of the jars it's tough to figure out the reason.
In Idea 6.0 it is very easy to recompile single classes including debug info and hot swap them for the original ones. If you have the source code attached to the library then simply hit F5 to copy the class to any of your projects source folders (without changing name or package) . Recompile. Hot Swap. Debug. Finally delete your copy.
This did not work in Idea 5.0.