CEENIC  pgpmail.bundle (b53) did not work initially after update to 10.5.3 from 10.5.2. setting bundlecompatibilityversion to 5 instead of 3 as stated in the installation instruction does the trick for me. just write in terminal:

defaults write com.apple.mail BundleCompatibilityVersion -int 5

and everything works again. voilą. haven't tried other values yet, as this works for me. hope this helps.  
(Version 10.5.3)

CEENIC  i did everything the readme states, i have the correct (legal) version for the upgrade and the software is installed in the default location. disabled everything as stated in the readme (yeah right, disable virus-protection (i.e. install ms office)). so apart from not having the ability of combo updates, the step by step approach doesn't seem to work either. another great throw from microsoft. 2 stars just because there is worse software out there, namely office 08 from which i downgraded again due to poor and sluggish behaviour.  
(Version 11.4.2)