Be wary of magnets & battery!
I have a friend who had a Droid (original), which also used the presence of a magnet to go into car mode. His battery life started going down the tubes when he got a magnetic-closure type holster for it (this was before he/we figured out what was going on).
However, he also experienced some MAJOR issues with his battery not long after. Meaning the battery would seem to do ok until about 40%, then start dropping like crazy. Juiceplotter would show it going from 20% to 5% in 30 minutes, no reporting by android in between, etc.
I didn't research it TOO much (wasn't my phone heh), but a cursory search showed some people had issues with magnetic cases and their batteries going to hell. I'm assuming that the magnet in the car dock is very weak and in just the right spot to trigger the car app, but a magnetic case (unless made FOR the G2X BY LG, so they would know about the issue and where to place the magnet) might end up having some effect on your battery. Or it might not. I'm no expert, but just check out the potential battery issues if you stick with a magnetic case.