In Management Studio 2005, the saved connections, usernames, and passwords could be cleared out by deleting the mru.dat file. However, in SSMS 2008 there is no equivalent data file. A similar effect can be achieved by deleting the SqlStudio.bin file. This will affect other settings, but for now it's the best solution.
In Vista, this file is located at:
C:\Users\{username}\AppData\Roaming\Microsoft\Microsoft SQL Server\100\Tools\Shell\SqlStudio.bin
And for XP:
C:\Documents and Settings\{username}\Application Data\Microsoft\Microsoft SQL Server\100\Tools\Shell\SqlStudio.bin
Aaron Bertrand has blogged a bit more about this
here.