The videos aren't cheap -- each track (iPhone, Mac, or IT) costs $499, or you can get all of the sessions for the entire conference for "just" $999. The latter collection of geek video features 146 presentations from the conference, perfect for those mid-winter coding marathons.
Apple is getting rid of physical media, so you purchase the videos, activate them, and then log into ADC through iTunes to download the videos and presentation slides. You need to be either a free ADC member or registered iPhone developer to activate the videos.
If you did attend WWDC 08, you are entitled to access the videos for free and should have received an email from Apple with instructions. If you've lost that email or never received it, contact ADC for details.