See the details at http://blogs.msdn.com/jeffbe/archive/2005/12/05/499980.aspx
I want to reemphasize, this is NOT a “go-live” release. You should not upgrade your production servers. This is a preview release for you to install on a VPC or other reimagable machine to see what changes we’ve made. There are lots of bug fixes, setup improvement, the friendly name DCR, etc. We eager to hear any feedback you have.
There is no upgrade path from Beta3/Beta3 refresh to this CTP or from this CTP to RC/RTM. There will be upgrades from Beta3/Beta3 refresh -> RC/RTM. This should reinforce the fact that this is NOT a production release.
We will have an updated “go-live” release within the next couple of months. That will be our first release candidate.