Got this today from them:
Brian,

On several PDA style phones there is a delay with any Bluetooth headset.
The Treo is the most pronounced, with a 3-5 second delay anytime you engage the headset button.

This is a function of the phone. The headset can only send the signal to the phone to answer the call, it is then up to the phone to feed the audio stream to the headset.

Hope this helps

I emailed back asking why the 640 and the 645 dont have the delay....