Microsoft initially planned to call next version SQL Server 2010. As they were working on it, they focused only on some aspects of it and due to the fact that most components didn't have significant changes decided to call it R2.l
That was a fair decision. I see only one problem with it. It will have a significant impact on adoption rate of SQL Server 2008.
Unfortunately, most shops waited until 2008 was released to upgrade their 2000 servers to 2005. Their rational for not upgrading to 2008 was that Service Pack 1 was not yet available. If Microsoft named next release 2010 as initially planned, everybody would hurry to deploy 2008. Unfortunately, most non-technical decision makers will not even notice R2. They will simply continue running their shops on 2005.