I created over the weekend a 'Trackball Answer' Phone apk forked straight from CM 6.1.1's Phone app.
For Froyo: Mirror 1. Mirror 2. MD5Sum: 36816def87f3c23ffd42f37a3b70e51b
For Gingerbread: Just get a recent Nightly. All CyanogenMod-based Gingerbread roms have my modifications and much, much more.
Flash the zip file from Recovery.
Source code is at my github, here.
I've tested this on both the G1 and the N1. Please report back if your phone isn't working, or this app makes your phone force close for whatever reason.
It's currently set to only listen to trackball events, but I know some phones don't have that and use a weird d-pad instead. If you'd like me to enable it through the d-pad, lemme know in the thread.
Package contains English, Spanish, German, Italian and French translations.
Finally, Trackball answer isn't on by default. To enable it, go to Settings -> Call Settings, and check the "Trackball Answer" options. I recommend "Double Tap to answer".
v5b a.k.a. Ye Olde Phone for Froyo
* Fixed and added Trackball Hangup options.
* Updated Spanish and German translations. (Huge thanks to xpatriick for the german translation and jrsoft for the spanish one)
v4 a.k.a. Rule 2.
* Added Double-Tap to Answer.
* Removed Hold-one-second-to-answer. It was annoying.
* Removed Tap to Answer. Current "Tap to Answer" means "Hold 350ms to Answer".
* Updated Eng, Spa, Fre, Ita and Ger translations.