Assuming tomorrow's Pro gets Light Peak (and Intel's coordinated announcement seems to confirm it), the port will make its way to the rest of the line relatively quickly. It would be a nice addition to the MacBook Air, and something that could lead me to replace the Rev D (I otherwise am planning to wait for the Rev F version).
Core 2 Duo/NVIDIA 320m combo to Sandy Bridge will be a slight upward move (faster CPU, slower graphics). That alone wouldn't be enough to push me to upgrade, particularly if the MacBook Air gets the Core i3 instead of the i5.
A Thunderbolt/Light Peak port, OTOH, would mean that people would no longer need to worry about the Air's lack of storage. External drives would be as fast as the internal drive. I could repackage my Crucial SSD from its current USB 2.0 enclosure to a Light Peak enclosure and get read and write speeds like my internal drive, and speed up backup operations to about 5 minutes from 15.
June might be a bit early. iPhone 5 is more likely for that month. July or August would be in time for the back to school rush.