Last month, I spoke at a SQL Server Firestarter event in Charlotte. With just about every session that I speak at or webcast that I do since 2006, I always ask for a show of hands on two questions. The first is "How many people here are running SQL Server 2000 in production?", and the follow-up question that I ask is "How many people here have SQL Server 2000 running in production for a majority of your servers?". This isn't scientific. People can interpret the questions/terminology differently (Does server mean physical box, instance, database, etc?), but something has changed - at least anecdotally. The responses for this particular audience were about 70% were still running SQL Server 2000 and of those about a little less than 50% had SQL Server 2000 as their major daa platform in their production environments. There was one guy there running 7.0 still (yikes!) and only 2 people in this audience running SQL Server 2008 in production.
This may only be a blip on the screen, but I find more and more organizations that are totally on SQL Server 2005. There is still a lot of 2000 out there and it will take time to make those a distant memory. But from this unscientific data point, it looks like SQL Server 2005 is on the verge of becoming the main version out there for SQL Server. There are all sorts of reasons why SQL Server 2000 is still in use such as a large investment in DTS packages, third-party apps, hassle of upgrading other components/applications/utilities. At some point, an upgrade will become necessary.
If you are in the process of upgrading from SQL Server 2000 to 2005 and can't (for whatever reason) upgrade to 2008 instead, go ahead and run the Upgrade Advisor for SQL Server 2008 too. Chances are that there will be little (if any) additional changes taht need to be made and you will for once be ahead of the game. :-)