Android - How do I turn off the cellular radio without disabling bluetooth?

Check out this guy's post on XDA: http://forum.xda-developers.com/showpost.php?p=11370203&postcount=2

This might be considered a risky solution, but if you aren't using the 3G connection at all and you're careful to simply rename the .apk's he references you might just disable the cell radio without needing to use Airplane Mode.

In case the link doesn't work, he suggests using Root Explorer to rename phone.apk to phone.apk.bak and telephony.apk to telephony.apk.bak thus disabling these two packages. You have to reboot your phone to finalize the changes. If it doesn't work, you should remove the .bak from each file name and they should be available again.

At OP's request, a summary of his findings:

Phone.apk is the Dialer app, which is responsible for the cell radio. PhoneTelephony.apk turns out to be just the Dialer storage, which we can kill just to save more memory. AndroidOS immediately restarts Dialer when it is closed, so renaming it will cause problems. But after a battery-yank-reboot, everything is ok. It also gets rid of the annoying Activate your phone dialog that comes up on every boot. btw, the dialpad still comes up after getting rid of Dialer :-)

Actually, the *#*#INFO#*#* (4636) method DOES work. While the control is greyed out under wireless settings, the taskbar icon still shows BT connectivity and I have tested and confirmed that BT is still active on my og Droid.