Thanks for that info I was driving around town today with my GS2 in MTP mode and connected to my car and I discovered that while it works, it only slightly works for some reason. The car is only reading a few songs, which repeat over and over again. In UMS mode, the car reads all my music (or at least the first 2500 songs since this is its limit).
I am REALLY hoping that this is just another case of my stupid phone acting up again and that this won't happen with the LG phone. I mean, hearing the same exact song over and over again makes you want to pull out your hair!!
EDIT: I just returned to the stock firmware (latest ATT Jelly Bean version) and it turns out that what I remembered about MTP is true: It doesn't work. So the only reason why MTP semi-worked was because I was on CyanogenMod. UGH! Now I'm worried about the new phone not working again!