It goes without saying that there is always something newer and faster just around the corner. Apple is known for updating machines every 3-6 months, usually near the beginning of quarters on the financial calendar. Best thing is to buy your machine when you need/want it. There is no "best" time for buying a Mac.

I can't really think of any solid Maya for OSX resources online. You can check the A|W forums, HighEnd3D, and maybe even CG-Talk for the occassional post regarding Maya for OSX. Not really a lot to it, though. Maya is pretty much the same on both platforms. The only exception is plugins which have to be compiled for specific platforms. MEL scripts, in general, work fine (sometimes a few simple modifications need to be made).