I'm not sure how much changes and features that they will incorporate in SQL2K8. But i would think they will just add up on top of SQL 2K5..  If you dont upgrade or at least learn sql2k5  before sql2k8, you may be left behind.  Remember there are tons of changes in 2k5 from sql2k.  I doubt it if sql2k8 will be totally different again.

- joriz