i've read the various comments posted here and the OP's frustration. I am also having similar issues whereas prior to PR1.2 I didn't. I am not running anything out of testing or dev repositories. i have virtually nothing on CPU load if at all when incoming calls exhibt this behaviour so it isn't that the device is busy either. as far as I can tell the phone application is playing up now since PR1.2 big time.
I had posted a [Request] for an enterprising coder to map Phone Answer to a hard key (camera key or other).
The camera key doesn't work well: Seems that when the display goes off, either rtcom-call-ui is unresponsive or that the camera button is disabled.
Anyway, I attach my quick hack. Untar, "stop shortcutd" (if it's running) and run a.out. Since I don't keep a SIM in the N900, I have no inclination to work on this.