There shouldn't be any problems whatsoever for whenever you choose to upgrade to a new OS. I will be waiting for a while after Leopard comes out to upgrade simply because I believe the longer it's out on the shelf the more stable it becomse. I will probably wait a couple of months to upgrade.

I would just get the Mac now. I had the chance of waiting for the C2D processors to come out, but I wanted my Mac asap so I settled for CD, seems just as good to me.

EDIT: Reason I voted for waiting was because I didn't realize you haven't had a Mac yet, I know my vote goes against completely everything I posted. Just get the Mac and wait a few months after Leopard comes out to upgrade.