I'll have to look into I2C. One of my friends who's into digital electronics has said good things about it too.
HopeRF's transceiver might be useful. How hard is it to use? I see that Low Power Lab has an arduino-like board to go with it and a software kit. Ending up with a dual arduino arrangement (one for RF, one for BT-LE) would be weird but strangely exciting. Though I'd rather keep the parts count to a minimum. How heavy is the processor load on an arduino with that chip?
Also, with what kind of terrain do you see a 900ft range? I'm planning to use this in places like large convention centers, shopping malls, and parking garages. I suspect all that concrete and steel could significantly decrease the range.
Another of my goals is to avoid unnecessary government paperwork. This is for a hobbyist project, not for mass production. 915 MHz appears to be in the unlicensed band but the FCC probably sets limits. I'll have to read up on the details before ordering too many parts.
Thanks for the pointers to I2C and HopeRF. That's useful information.