I think using twitter to track launch day lines would be a great idea. We could use a hashtag like #iPad311 or something similar to provide line updates.
I'm sure MacRumors, CNET, and Engadget will post line updates. However, they will probably be specific to cities like New York and San Francisco. CNET had a live blog of the lines for the Verizon iPhone 4 launch (or lack thereof).
I know I'll be updating my Twitter as I wait in line, starting at 7am