I don't think it's ever going to happen. First of all, the Air product line is threatened by the iPad, which, if it really takes off, will probably take away a large portion of the Air's intended market (people wanting light computing in an ultraportable package). Second, the Air is already criticized for having a large footprint, so it seems unlikely that this complaint would be addressed by going in the opposite direction of what (most) people want.
Unfortunately, it seems like your only 15" option is a MBP. I love mine, though I certainly wouldn't complain if it were lighter.