This has come up in conversations more than once in the past little while – recently on twitter I made the casual comment that later this year, SQL Server 2008 will be "two versions old." Well, not everyone agrees that that is technically true. So, I thought I'd put something out there that isn't limited to 140 characters. There are certainly some valid arguments on both sides, but my opinion – based both on these facts and on my memory that Microsoft has marketed it as such – is that SQL Server 2008 R2 is certainly a version that stands on its own. Still, I will present both sides of the coin (with my invalidation of the first set of arguments in green).
SQL Server 2008 R2 is simply an appendage to SQL Server 2008
SQL Server 2008 R2 is a full, independent version of SQL Server
Now, I will agree that 2008 R2 is a terrible, terrible, terrible name, doing nothing except make it *sound* like it's not a full version. In fact, I know that some people feel like they're getting raked over the coals when they upgrade, because they feel like they're paying five figures or more, per instance, for a service pack. (And given the features that were added to the core engine and tools, who could blame them for feeling this way?) It also causes a lot of confusion in the community, as I've alluded to in several of my recent posts about service packs and cumulative updates for each version – many people ask whether a CU for 2008 R2 is applicable to 2008, or if they should be able to attach or restore a 2008 R2 database onto a 2008 instance. My answer is always going to be no, they are different versions, and it has become second nature now to include a disclaimer in every single post.
So, with all that said, my opinion is still that 2008 R2 is a full and official release of SQL Server, and my statement still stands: sometime in 2011 or 2012, SQL Server 2008 will be two versions old. You may not think that way in all aspects of the product, but technically, I believe it to be true.
* There is one exception: if you've enabled 15K partition support in SQL Server 2008 SP2, you can't migrate that database to SQL Server 2008 R2 RTM.