For a limited time (first 100,000 downloads) you can obtain an updated Pre-RC1 build of the Vista beta (5536). Over beta 2 this has a decent performance increase, and a few nice touches in the MCE UI as well such as greater use of the thumbnail facility in My TV. This will also be available via MSDN. If by the time you read this the download limit has been reached, don’t worry RC1 should be coming in the near future.
Public Download: http://download.windowsvista.com/preview/prerc1/en/download.html