3 yearly releases is more or less the norm, as I understand it. That said, from Nov 2005 to Feb 2008 is actually only 2 1/2 years.

I believe from my reading so far that SQL 2008 will make the jump from SQL 2000 even more attractive, so perhaps the reason is that MS has found the uptake of SQL 2005 not strong enough, so they are trying to get SQl 2008 out to get people off of SQL 2000 (... and SQL 7). As I understand it, MS only support the current and previosu release, so when SQL 2008 is out, that will make SQL 2005 and SQL 2008 the 2 'supported' releases.