Thunderbolt on an MBA? Where would it fit? Would they have to take out a USB port to compensate?
The port isn't the space issue. As another poster remarked both 11" and 13" models have mini DisplayPort now. The external interface space is already present. Besides the "one USB" port was not a good design which the just recently corrected. To remove the correction is a step backwards (many folks are going to have a USB modem jammed into one of the ports. So still just have one free one in that case.).
What is somewhat more questionable is where the internal space is going to come from. The Thunderbolt controller is not a small chip (on MBPs it is 3rd/4th largest one on the board.). It is not as though there is copious empty board space on the current MBA versions. Apple got some reductions to squeeze it onto the MBP 13" version. However, for the 11" MBA not so sure there will be wiggle room unless some of the other support chips consolidate into smaller implementations.
Likewise pricing. The 11" at the inexpensive Mac frontier could be short ports just like the MacBook has been made to be. ( If the Macbook fades into the sunset all the more so for them to move that market segmenting technique over to the MBA 11". )