yeah I really wish it were possible to add an aircard to a minimac. I have it on my powerbook and i could use bluetooth to share the broadband connection between the two while driving but then I have to always take my laptop with me. kinda defeats the purpose.
anyway heres a forum from people who are using google earth in their cars, its a pretty long discussion but with good pics on page 5. one guy said internet speed doesn't effect it much and some are saying you dont need it all if you just scroll to the areas you are drivn while you do have internet it will cache it, I'm not so sure about that and I didn't read most of it but it mmight give you some ideas.
http://mp3car.com/vbulletin/showthread.php?t=63630
oh yeah I really like the minimac home theater idea too, i was looking at the tv tuners for them today. Too bad I sold google short last month, that stock really has to come down.