Spent a couple minutes playing around with the "Google Transit" tool. It's like Google Maps, but it shows you what sequence of buses will get you from point A to point B, as well as breaking down the ride times for each bus and when they arrive.
An improvement over the "maps" tool is that it seems to be better about picking up non-street-address locations. I punched up some directions for Seattle (Bay Area isn't supported yet), and asked for a bus ride from the Space Needle to Sea-Tac. The results are pretty good: it figured out where the space needle was, and that "SEA" was the airport abbreviation. It also can take an arrival time and back-calculate when you need to leave (although that's also obvious by the departure time of your first bus).
Now, if they wanted to make money on it, they should make it so you can do it from your mobile phone. Maybe call up a phone number, talk to an automated search engine, and have it send you back a text message with the instructions and itenerary.