I would buy now and worry about upgrading later.
Tiger is at least six months away (we'll probably get a better idea of exactly when at next January's MWSF keynote), so you'll probably be waiting quite a while if you don't go ahead and buy now.
Just keep in mind that the upgrade from Panther to Tiger will probably (we really don't know for sure) cost $129, just like previous OS X versions.
At the same time, Tiger will be the last major upgrade to OS X for quite a while. So, while $129 might sound like a lot, you probably won't have to shell it out again for another upgrade for a couple of years.