Just to let everyone know... I'm using a 3DR 900mhz air module with a 3.3V TTL serial cable.
It's the cheapest Chinese 3.3V TTL cable you can buy and it's working just fine. I've had some firmware update problems, but those happen on both air and ground modems, so it's not a communication problem with the cable.
I've tested the radio pair both plugged into my laptop and typed back and forth each way through com port terminal windows.
Hopefully this can save someone from having to buy an oldskool 5V FTDI style cable.
I sure wish they had put all the level shifters on the APM2.0 board instead of putting them on everything else. The processors on the APM2.0 are the only thing still using archaic 5V signaling AFAIK. The GPS, gyros, accelerometers, Si1000 radio, and everything else I'm aware of uses 3.3V.