Well, the simple solution is to have SPRecovery installed, which will block the OTA update from applying. Long answer is downloading a current custom ROM that's already running on a rooted 2.1 FW so the phone won't update.
Problem with the updates is more than likely you were running on either 2.0 or 2.0.1--when the phone updates OTA, if flashes to 2.1, EFFECTIVELY losing root capabilities, and wap/mms I know for sure, is out the door. I'm not sure about talk/text, but if you do a baseband update, you'll lose talk/text to. FW update and baseband update are two seperate updates. Talk/text fix is as simple as re-setting up the MIN/MDN/SID and reflashing the PRL. Wap/mms fix is done by writing NV items and pushing files, but you can't do that on a stock 2.1 FW because you need su access.