Treonauts, a forum covering the Palm Treo, reviews bluetooth headsets extensively. Check out the entire bluetooth headset section.

In there, you'll see that the reviewer ranked the Plantronics Discovery 640 very well for weight, but criticized its short talk/standby time.

I personally don't think it would stay in my ear, so I stuck with the Palm Wireless Headset. Though it breaks up when loosing line of sight, it's light, simple, and inexpensive. I DO have issues with the headset button not answering calls frighteningly often, which caused me to throw the damn thing at the wall (a few times), which stuck a button, causing me to have to open it up and repair it. Maybe the Plantronics IS worth the extra money...