If you are installing this update and encountering problems related to "System.IO..." errors or BPACMD.EXE execution problems or missing assembly BPAClient.dll then simply create a new folder called BAPClient in your
C:\Program Files\Microsoft SQL Server\90\Setup Bootstrap\BPA\
and copy BAPClient.dll from C:\Program Files\Microsoft SQL Server\90\Setup Bootstrap\Bin folder and paste it into newly created folder BPAClient.
Hopefully it should solve all these problems and successfully intall updates. Luckly it worked for me :)
More discussion on this issue:
http://social.msdn.microsoft.com/forums/en-US/sqlsetupandupgrade/thread/2a5f460b-8684-4510-8fcb-1e9d9786baff/