I recently migrated my email over to Google Apps, and after a few weeks everything is still running smoothly. The only issue I had was accessing the Google Apps Gmail account from my Treo.
For the rest of the world, you simply need to go to http://m.google.com/a and install it. However, it is not supported on the Palm OS. However, if you do have a JVM installed, you can install the app using the following URL:
Or the long version:
Thanks to GoogleSystem for the info.
The application works on my Treo 755p but isn't perfect. The font seems too big for the background layout. I'm not sure it is really much better than the pure web version though.