Here's how I found and tracked my error:
I searched: C:\Program Files\Microsoft SQL Server\90\Setup Bootstrap\LOG\Hotfix\SQL9_Hotfix_KB921896_sqlrun_sql.msp.log for
Return value 3
Here was the error that returned at the end of the log
MSI (s) (3C:C4) [15:14:01:390]: Product: Microsoft SQL Server 2005 (64-bit) - Update 'Service Pack 2 for SQL Server Database Services 2005 (64-bit) ENU (KB921896)' could not be installed. Error code 1603. Additional information is available in the log file C:\Program Files\Microsoft SQL Server\90\Setup Bootstrap\LOG\Hotfix\SQL9_Hotfix_KB921896_sqlrun_sql.msp.log.
And... when I searched for Return value 3 (with some digging) I found I didn't have SQL Writer / VSS components installed, so I manually installed those msi files, and then it worked correctly. Odd, but resolved. Basically, I made sure all of the proper stuff was installed (well, the stuff MS thinks is proper) and then it would install my SP2 for me correctly..