I signed up for Dopplr recently. Here’s the blurb from their ‘About’ page.

Dopplr lets you share your future travel plans privately with friends and colleagues. The service then highlights coincidence, for example, telling you that three people you know will be in Paris when you will be there too. You can use Dopplr on your personal computer and mobile phone. It links with online calendars and social networks.

The service has attracted a following among business travellers around the globe. Jimmy Wales of Wikipedia recently named Dopplr his “favorite non-wiki website” in The New York Times Sunday Magazine: “You put in your travel schedule and link to your friends. It allows you to see where everyone is. I love it.”

Their blog will keep you updated with their updates. An iCal/Google calendar feature looks quite handy.