We hit a heap space issue with Ant while trying to build a very large Flex project which could not be solved by increasing the memory allocated to Ant or by adding the fork=true param. It ended up being a bug in Flex 3.4.0 sdk. I finally figured this out after polling the devs for their sdk version and reverting to 3.3.0.
For the curious.
I tracked the bug down to an Interface file that had an additional accessor pair added "get/set maskTrackSkin". The heap space error hit if any additional functions were added to the interface and to make things worse the interface was not in the project that was getting the heap space error. Hope this helps someone.