I have submitted a pull request https://github.com/diydrones/ardupilot/pull/520 that adds Spektrum satellite support to the APM2.0/2.5. I know this was possible with the old APM1 and I've seen some people requesting this for the new APM.
I have tested it quite a bit on the desk, but not in flight, since I haven't completed my UAV yet (I wanted this to work first so it will be less of a cable mess). You are free to test it but I take no responsibility if it makes your UAV crash. This is my first contribution to an open source project so I'm not 100% sure about the workflow, but I thought I'd post here as well as adding a pull request, and hope that the right people can test it and verify that it works.
Note that you need some sort of adapter because the satellite is designed for 3.3V, a simple regulator should be enough, since 3.3V is still enough for the AVR to detect as high. I use a fancy adapter that a colleague of mine built, it allows me to connect 2 satellites for diversity and merges it to one output.
Connect a jumper between channel 3 and 4 to enable this mode. It currently does not work with 11 bit frames, and you may experience servo glitches when you have the the USB cable connected. See the pull request for more info.