Here is how I addressed this using Toodledo's SMS and alert function:
http://forums.omnigroup.com/showthread.php?t=12624
I concur that OF needs to have this capability in both the mac and in the iPhone. When iPhone 3.0 comes out, it should be a breeze on the iPhone. It is straightforward to do it now on the mac.
Those who claim the purist mantle and say "do that on your calendar" don't have to use it if it's there. In point of fact, put a check box in the preferences that says "Don't supply any notifications" or something like that. Don't like it, then don't use it. To say to not have it is basically a ridiculous argument and probably big time limits the market that OF tries to address.
There are quite a number of actions that need both a context and a time to be enabled.
J.