Before I went Froyo, I had done three factory restore/wipes on 2.1 withot a single issue to my 3g speed/ability. Then I rooted (the old way, in&out with the SD card), then 2.2 came out and I flashed all the way back to 2.1 stock with a wipe and factory reset... 3G was working fine. I installed the OTA Radio, 3G was fine. I unrEVOked my phone, 3G was fine. I installed 2.2. 3G was fine. I messed with the build.prop changes for audio and other changes for audio on 2.2 and had to wipe/clear and reinstall Froyo from update.zip three times. 3G radio is STILL working fine.
I've never seen the problem, but then again I am one user on one phone. YMMV/Grain of Salt/Good luck.