I was afraid something like this would happen. Garmin seems to be locked into hardware-only solutions. My guess is that the Pioneer lawsuit had something to do with the decision. I just bought GMPC about 6 months ago - argggh!
I'll just have to go back to iGuidance I guess - inferior to GMPC in my opinion (maps aren't as accurate as GMPC for North Texas - although the GUI is better for CarPCs).