After activating the device encryption on my S2 running CM 10.2 nightly, my CDMA radio stopped working I have a Samsung Galaxy S2 (Sprint, codename d710), and I just installed the latest CM nightly (10.2). Everything worked until I encrypted the device. Now the radio doesn't work (network: UNKNOWN, no service, etc.). I looked at each of the mobile network settings and didn't find anything; I also tried about 10 different modem updates (packaged as .zip, installed through Clockwork Mod).

I verified that it happens immediately after an encrypt from a fresh install of CM nightly 10.1-2013-11-17.

Is there anything to do short of wiping and reinstalling? What's the source of the problem, and how can I prevent it from happening again?
Cyanogenmod nightly builds are development snapshots. Anything can be broken in them from one night to the next. You shouldn't try to use a nightly for your "everyday" phone as problems like the one you describe are common.

Report the bug to the developers and switch to a stable release (or, if you're feeling brave) a release candidate or milestone release.