Recognize that Feb 27, 2008 is the "Launch Date". The release date is not the same as the "Launch Date". That only means there will be big events, satellite broadcasts, and boxes in stores on that date.
Visual Studio will still be "released" by end of the year: http://blogs.msdn.com/dseven/archive/2007/07/10/windows-server-2008-visual-studio-2008-and-microsoft-sql-server-2008-joint-launch-announced.aspx
SQL Server 2008 is still planned for a "2Q 2008" release (that would be after the "Launch date"): http://blogs.msdn.com/dtjones/archive/2007/07/10/sql-server-2008-to-launch-on-2-27-2008.aspx
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.