Call me overly dramatic, but I really think one way or another, this is going to be an historic release. Not necessarily for quantities or crazy lines or people jacking up prices on ebay, but because it's really a rare and unique case for a tech device to essentially release into (and therefore create) a brand new space. The iPod, while obviously a major tech milestone, is/was still initially a portable musical device, replacing conventional Sony Walkman and clones. It had a clearly defined space. Likewise the iPhone entered the smartphone market; it certainly re-defined the space, but it was a clear market that consumers understood.
The iPad has a much more difficult task at hand, as there's really no true definable precedent...