According to
darwinx86.org, the future Mac OS X update, aka version 10.5.7, will not only include minor bug fixes and improvements, it will bring support for the latest Radeon HD 4850 et 4870 and for the forthcoming Intel Nehalem processor.
So, one could even predict that it is built for the future Mac Pro, expected to be powered by the Xeon Nehalem CPU that Intel is supposed to officially announce at the end of March. So, we should then expect Mac Pro models update to come couple of days before or after this date.