It has been out for a while now, but I just got around to installing the push email update for the Treo 650. I had to upgrade Versamail, so it cost about $12 total to get it working. Once it was installed, it was really easy to connect it to my Exchange account and the push email works, as well as calendar synchronization. Pretty cool. It has crashed the Treo once so far, but it crashes a lot, so I wasn’t too surprised. Also, if your exchange server is heavily firewalled, like my previous employer, you may not have much luck, I imagine.
Anyway, get it here.
I noticed they also have Blackberry connect for the Treo 650, which probably achieves generally the same thing, but I bet you have to do more work on the server side, plus something on the carrier. This required nothing on the server and works with a regular data plan.