I'm betting they won't be available for individual purchase. Although the appeal of MxM and mobile PCIe graphics is the upgradability factor, the fact that no one is currently offering upgradeable mobile PCIe graphics after almost a year of availability leads me to believe that demand probably isn't high enough, especially since most end users don't know enough to open their notebooks and do an upgrade.
In addition, since most notebooks' GPU cooling systems are proprietary and designed specifically for the GPU that came with the system, this further complicates upgradeable mobile graphics. This certainly shouldn't be a problem with the 6800 Ultra and the 7800 GTX, but upgrading from an X300 or 6200 to an X700, 6800, 7800, etc. would undoubtedly present cooling problems. Futhermore, I bet most notebook OEM's/ODM's will void your warranty on such an upgrade unless you buy the part (and probably some service fee) specifically from them (like Dell did with the original Inspiron XPS and the post purchase option of upgrading to a 256MB Mobility Radeon 9800, which had to be purchased from Dell...with a service fee.. and installed by Dell, IIRC).
Anyhow, I'd love to think resellers would offer them (albeit at reasonable prices), but I don't see that happening any time soon.