SQL Server 2008 transaction log growing since SP3 Installed
I'd suggest the 2 are unrelated.

Has anything changed:


in the maintenance plan setting
recovery model of the database (is it now FULL where it was SIMPLE)
did you take log backups before (FULL) in the maintenance plan? Do you still do them?
did you have auto shrink in the maintenance plan? This is bad anyway
have you any open transaction that is preventing a log backup (or shrink)


Also, restore a database from pre-SP3 on another server and compare the sys.databases entries