Would a dedicated graphics card fit in an 15" MacBook Air form factor?
I think that is the wrong question.
Does it make sense to do it.
Would somebody who wants to play games and wants more then the Intel IGP provides buy a 15" MBA. Probably not?
It is for people that want a normal work machine but with a bigger screen. In OSX hardly anybody needs dedicated GPUs.
Secondly what GPUs might you be able to fit in. There are those Sony notebooks that almost always sport dedicated GPUs in the entry range (like a 520M). Those still have a TDP of some 10W but are a complete waste because they are not faster than the Intel HD 3000/4000 by any significant margin. You'd need 15-25W to get a GPU that really is worth the effort.
AMD doesn't plan on making any more 64bit GPUs because they are not worth it anymore as they cannot truly beat the IGP solutions anymore.
People still hold on to the myth that Intel GPUs are soo bad. Yes they used to suck but they don't anymore. They run anything on the Desktop without any problems only GPGPU acc. apps or Games can really use something faster. The current Intel GPUs are much faster than the 9400M but back then nobody complained about the 9400M but Intel is still bad. Yet Animations, Videos ... stuff that you do out of Games didn't change at all since then that stuff doesn't need more speed than it used to. Only Games do and some GPGPU Apps which can never really have enough speed.
For all the things I do on OSX. Work(programming), movies, music, typing, websurfing the Intel GPU of Arrendale which is half as fast as the Intel HD 3000 does everything fine and without any choppyness. Without an external screen I would for heat and noise sake never switch to the dedicated GPU.
I still need it for Windows and my occasional gaming thus I wouldn't by a 15" air as this is my one and only computer.
Would I have a desktop and run pretty much only OSX along with some VM Windows for special apps on my notebook such a 15" would do just fine running on Intel only.
I believe they will not even put a quadcore into a new 15" MBA until Ivy Bridge. Because most people really don't need those. If I think back on my student dorms it would have been a waste for almost all the students. Dual Core is just fine. I will upgrade eventually because I really would like to speed up compile times. That sometimes takes so much time.
I think google/amazon should provide a compile webservice.