I ordered a Seidio extended battery from the Treonauts store, luckily they were in stock, currently they're sold out. I received my Seidio extended battery for my Treo 650 today. It arrived with a 40% charge, and it's currently charging now. The reason I bought another battery, was Seidio just came out with a new version of their extended battery which they claim provides approximately 33% longer battery life. The battery will also charge in my Palm Treo Cradle Kit, but I plan on keeping this in my Treo, and keeping the original battery in there charging for backup. I use my Treo a lot during the day, to update the site, IM, web browse, and email. Chatter email is great, but I have it checking emails constatly (I could set it less frequent, but I like getting my email once it gets in my inbox), so it takes a toll on my battery life. This should definitely remedy that.
I've read good reviews on Treocentral about this battery, and will give an update after a few days. It should also work great in the upcoming, and my future Treo 700p, since it works with the Treo 700w as well ;)