Earlier this week, a hotfix was released that, among other thigns, addressed an issue where, if you restored a 2000 database from 2005 SSMS, it did not go well. The KB article (KB #972687) says that only certain versions of SSMS are affected, and proceeds to list build numbers that are from cumulative updates, not major service packs. As you may already know, Help|About in Management Studio only reflects the version of the last major service pack (or RTM), unless a cumulative update touches AppIDPackage.dll (which is apparently quite rare). So even though changes in CUs (such as the one that caused this specific problem) update code that SSMS uses, SSMS itself does not indicate that it has been updated. This was complained about in Connect #275512, which was incorrectly closed as not reproducible, even though Buck Woody openly admitted that this is true in both SQL Server 2005 and 2008.
At the urging of myself and a few others, Bob Ward has published a very comprehensive blog post about the restore database problem, and how to truly determine the version of SSMS you have installed (at least, if you know which file(s) you need to check; also a rare occurrence). Rather than repeat what he wrote, I'll just link you there:
http://blogs.msdn.com/psssql/archive/2009/08/21/ssms-fails-to-restore-with-sql-2000-instances-and-ssms-versioning-explained.aspx