I have a 600 and installed CallRec. I checked to see if I have the latest update from PalmOne and I do (Treo600-1.12-INT). CallRec is still not working. Why not? Here is what the announcement said a while back about this, but CallRec still does not seem to be working with my 600:

Free voice recorder released

Sun May 16, 2004 - 7:52 PM EDT - By Marcus Adolfsson

For those wanting to test out the new recording capabilities of the Treo 600, Infinityball has released SoundRec - a free voice recorder.

From their site:

SoundRec v1.01 has been released. This is a simple sound recording application that allow Palm OS5 devices to record simple voice memos. It will work on Treo 600 smartphones that have been updated with the latest firmware that adds support for recording audio, you can get the firmware update from PalmOne. The latest version fixed a bug that would cause the data to become corrupt when the app lost focus

Infinityball also recently updated their camera recording software MovieRec to support voice.