I have a vastly improved development version running on my N900 that has most features that people requested (larger icon, clear recording indication, configurable save folder, configurable recording format, configurable recording mode [dictaphone, voice, bt]). It has callbacks for going offhook and onhook when being called over GSM. It just needs some more internal testing, sorting out of some glitches and adding auto record on outgoing calls.
Testing is just a nightmare as I have to keep calling myself from my landline :-)
(or even worse, call my own landline in the evenings - the wife is not pleased :-)