It just doesn't make financial sense for GM to do this now.
For starters, they are already running a standard OS as their backbone; as such, their equipment isn't designed to handle Vista.
More importantly, however, is that for such large companies, migrating to a new OS backbone would mean huge upgrades across the board; that requires resources in time and money. If GM takes the upgrade path now with Vista, they'd be shooting themselves in the foot knowing that Windows 7 is only 1-2 years away.
Windows 7 promises to be meaner and leaner than Vista. For any company the size of GM who hasn't already migrated to Vista to do so now would be utterly asinine, especially knowing it could take a year or two to complete a migration and ensure everything is working without kinks. By then, Windows 7 would be out, and GM IT would be back to square one without even having exploited the Vista migration.
If anything, their IT team / director is making the right choice.