Quite interesting - more than foursquare maybe. I don't really care where people are right now - unless they have just arrived and I can still join them. In any case, that's a short window of opportunity.

However knowing where they plan to go means it'll be easier to plan in advance and join them.

Facebook and the calendar / groupware software already made that possible, but not so easy. Hopefully forecast will fix that.

No API however.

Next: status updates from the future, then calendar sharing.