There is documented evidence that there are Rentals coming to iTunes. Strings have been found in The iPhone, itunes, and the iTunes store as well. These all point to a Wi-Fi Store on the Apple Tv. Apple had been a little bust lately with the iPhone, leopard, the new iMacs, Ilife, iWork, the new iPods, and there entire professional video and audio editing applications. Many people are expecting an Apple Tv update, although it might he more software based rather than hardware based. Jobs himself said back in the middle of the year that an apple tv announcement was "coming soon" and that has yet to appear.
In short, I would wait until macworld. Although remember that the largest difference likely to occur Is going to be in the software, so all owners will be eliagable, like how the iPhone updates are done.