I jumped onto the MacBook Air bandwagon (and actually back onto the Mac bandwagon, for that matter) 4 years ago when the MacBook Air was first announced.
What's ironic is that back then, when it was released, the Mac message boards (I frequented Macworld more back then) were highly dismissive of the MacBook Air and claimed it would be a flop. Remember the hype back then was about a "magic" docking station, and the introduction of a notebook with no ODD and only 1 USB port, and no docking station was somewhat underwhelming.
However, I had long been a fan of ultraportables, and once I saw Steve Jobs embrace it wholeheartedly I knew back then that Apple would stick with it until it got it right. Even the first MacBook Air, as crippled as it was by the slow HDD (I wasn't about to pay for the $3000 SSD model back then) and integrated graphics, refused to compromise on screen quality and a full sized keyboard. A few, particularly Jason Snell at Macworld, were on board right away just like me, but others dismissed it as the next Cube. Sales were relatively slow at first, and its success wasn't guaranteed. Heck, right before the 2010 MacBook Air was released there were rumors that Apple was going to drop the Air.
Already HP and a few others have announced or hinted at 14" and 15" Ultrabooks, and Intel even has some spec requirements for them, so it would not be surprising at all for Apple to expand the Air line. Remember, too, that the 2010 MacBook Air's tagline was "The Next Generation of MacBooks."
I think the 17" Pro may be the holdover that retains the ODD and HDD a bit longer. Remember, it is the only MacBook Pro that still has an ExpressCard slot. I think the 13" Pro will likely go the way of the white MacBook once the Ivy Bridge chip is released, particularly if Apple is able to fit a quad-core into the Air's form factor. The 15" is interesting territory, though. It might retain the discrete GPU, and possibly even the HDD, though I wouldn't be surprised if it went all SSD, or at least primarily SSD.