What do you need to attach? Remember that people buying an ultra-light (I ordered my Air today) are trying to slim down on the computer and likely on attachments as well.
The Air has a multi-touch trackpad: leave the mouse at home.
Printing can be done wirelessly.
While out and about, the only thing I will need to attach to my Air is my camera so I can put photos and video clips to use immediately (my camera only has USB, no FW, so no problems at all; it also has hi def video!). That comes to a grand total of one USB port that I will need.
If you really need two USB ports, you are already lugging a couple of extra things around, so a tiny USB hub wouldn't change much. You could always buy a MB or MBP if you need more power. The Air is about lightness and slimness. All excess is cut.
Say for example i'd like to charge/sync my iPod for a couple of hours and burn some work on the external DVD drive, or print, or access an external bus powered HDD, or use the ethernet adapter: not everyone is going to be able to afford, or have immediately available to them, the wireless house of the future complete with TimeMachine, wifi, wifi printers, remote DVD drives etc etc...
Just seems like compromise and assumption to me.
Bus powering/charging isn't all that great over USB hubs, unless you buy a powered hub which will need a PSU, which wouldn't really do the crisp aesthetic any favours.