Given that I tend to travel a good bit (and expect that to continue with most of the jobs I might take), I often find myself sending messages like the one I sent today to a group of people saying "Will any of you be out at the Podcast Expo next week?" Some of them answered... for others it was just an another annoying email in their inbox.
Wouldn't it be nice to simply know where your friends/colleagues are? (Assuming they shared that information with you?)
One approach is what you see with Facebook Events, where people can RSVP into the Event inside of Facebook. You can then simply look through the list and see who is there. Upcoming.org works in a similar fashion as do probably half a dozen other services.
The challenge is that they are all about events. But what if you are going to be in Boston for one event and a friend/colleague is going to be in Boston for another event? By these event-driven sites, you won't necessarily know that.
Dopplr, on the other hand, is location-driven. When you enter in a trip you are taking, it ties it to a standard geographic location. The advantage to this is that you can then look in the city you are going and see: a) who among your circle of friends lives in the city; and b) who will be traveling to the city (provided they are sharing their trip information with you. From their "About" page:
How does Dopplr work? It lets you share your future travel plans with a group of trusted fellow travellers whom you have chosen. It also reminds you of friends and colleagues who live in the cities you're planning to visit. You can use the service with your personal computer and mobile phone.
It's an interesting idea to me, given how often I'm going places. To be useful, of course, it needs to have a certain mass of people using it. If you do use it, my URL is http://www.dopplr.com/traveller/DanYork If you would like to try it out, drop me a note and I can send you an invite.
P.S. Congrats to the Dopplr team for receiving early stage funding from an impressive group of investors!