If you haven't checked it out yet, do check out Backpack. Backpack is a new personal and small biz information management tool that, among other things, allows you to set "reminders" that can be sent to your Treo (or any other mobile phone that accepts SMS) at a time you specify. It's quite useful.

Find out more about Backpack at
http://www.backpackit.com

And here's a video of the reminders functionality
http://backpackit.com/example_movies/reminders.mov