I just had a cool idea, though it may not be original (and it may not be feasible with current cell phone technology).
My Treo, like any PDA or smartphone, currently lets me create an item on its calendar and specify that it should give me a reminder alarm a certain number of minutes (or hours) ahead of time.
Which is fine, but sometimes what I really want is for it to remind me to do something when I arrive somewhere. If I leave work and start to head home and then realize there's something I want to remember to do as soon as I get home, I can certainly pick a time n minutes in the future and create a calendar item with an alarm on it for that time; but it would be more elegant to be able to say "When I get home, regardless of how long it takes me to get there, remind me to take the trash out" (or whatever).
This would require my Treo to know my location. I'm not sure what the state of the art is for integrating GPS into cell phones. But I keep thinking that in a few years, GPS will be a standard part of any cell phone, and then it seems like it would be fairly straightforward to write an app that says "When you get within n meters of location x, set off the reminder alarm."
Do GPS units already have that capability?