It's weird, I've been in a few situations recently where my friends and I ended up in the same place at the same time without either of us knowing we would be there. Like just last weekend I was on vacation in Tahoe and I ran into a friend of mine who was staying at the hotel across the street! But since we had already planned out our activities for the weekend, we couldn't find a space in our schedules to meet up. I was kinda bummed.
So what makes Dopplr cool is that you can upload all your upcoming trips, and then Dopplr checks your Facebook, Gmail, Twitter, Flickr, and LinkedIn friends to see if they are on Dopplr too. That way you can see which of your pals are going out of town and when, so that if you happen to be in the same place at the same time (hopefully in a place like Hawaii), then you know ahead of time and you can meet up for drinks.