sub0: Your cell phone should work but you may have to manually arrange for the ppp connection to point to /dev/ttyUSB0 or /dev/ttyACM0. The ttyUSB0 might need the airprime and/or usbserial kernel module. ttyACM0 uses cdc_acm. Note that the cell phone might request enough power so as to require the external powered hub. Also some of these modules have acted strangely, but I haven't had time to recompile everything to verify it. So the answer is that there is no technical hardware reason the USB cell phone hookup would not work (allowing for power). The software might need some tweaking to get everything to work.

bilofsky: The n810 detects host mode by a plug which shorts pin 4 (sense) and pin
5 (ground) together. The stock cable doesn't do this, nor does it bring out pin 4 to where it would be accessible. You can however do it like they do on the n800 where you use a shell script to switch between OTG/Peripheral and host mode in software. The shell script mode switch trick does work on the n810, then a female-to-female adapter would work as you said. I actually have several M-F A extension cables in pieces and I tried it this way before I got the Motorola adapter.