My answer above caused a discussion within Alfresco about supported versions.
The official JDK for Alfresco 3.1 is 1.6 Update 13
So when 3.2 comes out it will be at least a 1.6 version.
But the 3.X code lines are all written to the 1.5 APIs and no use is made of 1.6, hence my comment above. However since JDK 1.5 is now about to reach its end of life it seems likely that the code line will move forward to 1.6 fairly soon.
That would be a good thing in my opinion. Most other products are up to 1.6. Integration can be a real pain if you have to keep multiple API versions coordinated.