Microsoft just released the July Community Technology Preview of SQL Server 2008 (codenamed Katmai) and plans a new CTP approximately every 60 days instead of a formal beta testing process:
The company is using the CTP process instead of a more formal beta test cycle for SQL Server 2008. By the schedule, a third CTP is targeted to ship roughly 60 days from now, and that would put a fourth CTP at about the end of the year.
Of course, the ship date is somewhere in 2Q08 despite prominent participation in a goofy launch event next February.