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SQL Server 2008 R2 has been out for a while now (see http://www.microsoft.com/sqlserver/2008/en/us/R2.aspx) and many people are trying it out.

One word of caution: any database attached to a 2008 R2 server will have the database physical version number increased to 661. The database version number for 2008 RTM and SP1 is 655. This means that you won't be able to take that 2008 R2 database and attach it back (or backup/restore it back) to 2008 RTM, SP1, or SP2.

I don't mean this as a reason to not play around with R2, just be careful you don't inadvertently upgrade a database that you still need to run on 2008 RTM, SP1, or SP2.

For more info on database version number and how previous versions are not forward-version compatible, see these blog posts:

Hope this helps!