I think many companies dive head first into unlimited vacation policy without really thinking things through. I had a job where we had it and it somehow worked (despite the CTO's inability to follow any kind of project management structure).

Our PTO's were scheduled ahead which was policy. So if you took a two week trip, you filed it three months in advanced. You let your team, your PM, and business know you'll be out of town so they can manage expectations (e.g., less work). Our sprints were based on how much work we can do based on the existing resource. My team had a team of 3 so we never scheduled PTO's were two or more people were off for any prolonged period of time. Sometimes schedules collide, but we do our best to be transparent about big trips so that we can all plan accordingly.

You know, it worked well. I'm glad it worked well for you at Trulia (which, btw, helped me butt loads when I was shopping for a home, so thanks!). I'd work for you guys except I'm not a mobile dev, I like where I'm at, and I'm not looking to move. :)