It is known that you can go up 2 major releases. Let’s consider 2008 R2 as a “half release” as we can “floor” 10.5 to 10. That is because we can go from 8.0 (2000) to 10.5 (2008 R2). Microsoft is smart as it is allowing several costumers running SQL Server 2000 to upgrade to 2008 R2.
But there is a caveat, you cannot upgrade from SQL Server 2008 SP2 to 2008 R2 RTM if you have the option to support 15,000 partitions enabled on 2008 SP2. So you are pretty much stuck until Microsoft releases a CU or SP to address it in R2.
Make sure you test before upgrading a production environment.