I was at WWDC, and I can tell you that if you are serious about developing for the iPhone, buy these videos. They are full of practical information, especially regarding the new additions of iPhone OS 3.0 and OpenGL ES 2.0 to iPhone 3GS (and now the new iPod touch). The presenters are much more polished than you usually see at technical conferences.
For example, I wanted to implement custom copy and paste support in my application, so I watched the video, and after an hour and a half everything worked for me. If I took the time to search for the right information on the Internet or in the documentation, it would take me a lot longer than the simple “this is what you need to know” approach at the WWDC session.
The videos are beautifully encoded for playback on your iPhone (or iPod), so you can take them with you to watch on an airplane or while sitting outside.
If I'm not mistaken, you also get access to some sample code that was available only to WWDC participants and has not yet been published on the Internet. In general, you get a lot of content for your money.