A dude from Dopplr has posted a presentation on how it works.
"Made of Messages" by Matt Biddulph explains "It's important to think about serverside architecture as an asynchronous system." It then explains how the internals of Dopplr work very asynchronously. Good stuff.
The part that really caught my attention is slides 6-8, where Matt recommends reading Enterprise Integration Patterns in order to understand asynchronous programming and messaging better. Matt says, "This is a great book. Really. Ignore the name." (Apparently there's a problem with the name?)
So if you're busy doing everything all Ajax, Web 2.0, mash-up and what all--hey, it turns out that Enterprise Integration Patterns is still a good book for you, even if you don't think of what you're doing as "enterprise" or "integration." Thanks, Matt. (And thanks to my friend Andy Piper for letting me know about Matt's presentation.)