I finally got around to converting my Login Script, written in VB, converted to use Group Policy Preferences. The conversion went quite well, however a few cases did show up where some Windows XP computers were not mapping the drives.
After some investigation, I was able to determine that this was caused by a missing Windows Update. A few of my computers that were not imaged correctly had the same SusClientID used by my Windows Update Server which prevented them from downloading updates and reporting status.
To fix the drive mapping issue, I simply downloaded and installed Group Policy Preference Client Side Extensions for Windows XP (KB943729). After rebooting the users were able to map drives without a problem.
I then went on to resolve the matching SusClientId issue.