I thought I should write a short post on this issue. It had been discussed before on forums and in bugs. Long story short, VS2005 SP1 does update all VC++ redistributable MSMs in \Program Files\Common Files\Merge Modules. Or I guess I should say, it is built to update them. However if it did not update them or some of them, there two primary reasons for that:
1) Something went wrong in SP1 installer and it never reached a point when it installs these files. Check installation log for SP1 and repeat install. Sometimes you may need to reinstall VS2005 also first.
2) You might have used Orca to view content of RTM version of MSMs. Orca has this "feature" of modifying timestamp of MSM when it opens it, even when no edits are made to content. To workaround this "feature", VC is marking MSMs as read-only in Orcas. Of course, if you have changed timestamp of the file by hand, you are going to see the same behavior.
Usually uninstalling VS2005 SP1, repairing or re-installing VS2005 and then installing VS2005 SP1 again fixes all problems including non updated MSMs.