This type of upgrade would be very nice, in general. Better battery life on the air would be great (not that the battery life is bad, but more is always better). More than that, a faster SSD would be better, as I've been reading that the mac SSD's are not really that fast compared to other SSD's (but that's just what I've heard... I haven't bothered looking up the numbers yet, but I've read that claim many times). Of course, the current SSD is way faster than a disk drive, but still, faster is always better! Especially when you're charged the Apple tax!
My only concern is having the drive memory soldered into the computer... it makes repairs very very expensive.... and the air is already very expensive to fix if you have a SSD problem. But I suppose that's what applecare is for... 