The 2010 Mac Pro is basically a 2009 Mac Pro with some new CPU, memory, and drive options. But the underlying technology is basically over a year old. With a simple firmware update to the 2009 models, you can have all of the same options available for last year's Mac Pro and probably hit the same benchmarks. I'm hearing that "sandy bridge" will usher in a whole new platform, and that will definitely be worth switching for if you need raw performance.
With that said, a 2009 Quad is basically a 2010 Hexacore with two fewer cores. For 90% of the applications out there, there will be no difference in performance. If you see a good price on a 2009, I would snap it up and wait for Sandy Bridge. The 2009 platform was brand new for "Nehalem", the 2010 is essentially the same platform and therefore an interim solution until the next major platform change next year. People said the 2009s were a bad deal, but considering the 2010s are nothing more than 2009s with slightly changed accessories, it's the 2010 that I think is a bad deal. The 2010 Mac Pro is riding on the coattails of the 2009, as it is basically the same hardware, and is a stop-gap product until 2011.