I've been unhappy with my new laptop since I got it in May, but I just did the update to 10.6.4 and jME is once again running GREAT!
As some background, the problem seemed to be prevalent in Nodes which had multiple meshes with their own TextureStates under it. But texture performance in general was rather poor before the update. After the update I'm now getting almost identical framerates across all three operating systems 
I figure this is a bit of good news that us Mac users were really needing!
Good to hear!