I really like the idea, but the feature mentioned at the end of the post to allow limiting plans to certain friends is a must have - many of my plans I won't want the whole world to know about.
We have been going back and forth on how to do this, the problem is we really don't want to clutter the simpleness of "what, when, where" with lots of other dialog boxes, so we are trying to find the best way to integrate advanced functionality like privacy and still remain simple.
very nicely done, we tried something similar some time ago but i never finished it :-) good luck