It's actually not even just shortest distance -- it's some combination of that plus having information about traffic. I had one comical trip from Arlington to MIT, where Google *knew* that Mass Ave was one long traffic jam but damned well wanted me to take that anyway. I took the shorter (and less trafficked) back roads instead, with GMaps screaming at me the entire time to turn onto Mass Ave, despite the fact that, every time I ignored it, its own estimate of the time to destination got shorter. Far as I can tell, this was because it didn't *know* the traffic on the back roads, and was therefore unwilling to route me on them.

I love GMaps in many ways, especially for the real-time (and predictive) traffic info. But they need a lot of work on the routing algorithms -- they're still not as good as my little Garmin...