Actually they have.
http://www.apple.com/pr/library/2005/apr/12tiger.html
I'm calling BS on the prepaid though. I highly doubt that Apple and AT&T will make the iPhone available through the GoPhone program.
o, woops - thanks for the corrections! I feel kinda dumb now 
after thinking about preorders though i really think that it would be a bad market move for Apple, because they would increase the secondary market price before the imminent launch. people would price gouge like no other when reselling their preorders.
this is already happening on ebay as its been posted already on this forum... kinda ridiculous if you ask me.
going back to preorders with OSX, a slight difference between that type of preorder is the production is a lot LOT easier and less costly to ramp up before launch, whereas the iPhone is a piece of hardware (as opposed to software being burned onto a disc or discs)
and quite sophisticated.
there is a set, or fairly set, number of handsets that will be available at launch and although that number is unknown, the fact of the matter is it is finite.
EDIT::
omg! 9th post! 1 more and i get a chance to win a free iPhone!