For me, a new release of MS SQL Server every 24 months is far too frequent. As already mentioned in this thread, upgrading from 2000 to 2008 is entirely possible, and at this moment in time, probably sensible.
So this gives us a real-world upgrade period of 96 months! Not what Microsoft would like to hear. Even without the problems of vendors not supporting the latest releases, the main question for a business is: does it work at the moment, and what improvements will we see if we upgrade?
Both 2000 and 2005 have proven themselves to be good, solid, reliable database engines. Why upgrade? Or, alternatively, why spend time and money upgrading?
Admittedly the latest version has better management tools, but nothing that fundamentally improves the DB engine.