I agree with Steve, a regular Service Pack, every year for the supported life of the product. That would mean Service Pack 4 is that last for SQL Server 2008. You can live with that, without getting to SP5a (remember SQL 6.5). It also gives you an upgrade schedule and some known certainty, without having to lobby for a service pack (SQL 2005). The wider implication would be not just for SQL Server but for every Microsoft supported product.