Actually we planed to support JRE6+ only, but so far Mac OS X has JRE5 only (it is said to have JRE6 soon, but who knows) , so we changed the plan.
I just check the source code, fortunately most code are compatible with JRE5, but the performance goes down a lot, I heared that JRE6 provides a lot of optimize in runtime, and now I believe it is true.  Anyway, we will optimize the code to improve the performance, we hope ForeUI could work well in both JRE5 and JRE6.