Its difficult to tell, look for laptops with MXM compatability, they recently changed from the old MXM 1/2/3/4 system to MXM A and MXM B i believe, where MXM B can hold the largest highest performance cards and MXM A smaller cards.
But even if you have a laptop with something like MXM B which will typically come with anything between a GTS 250 and an ATI 5870 in it already theres no guarantee something released in the future will work an ATI 6870 for example may have increased power requirements/thermal requirements and on top of that getting a hold of an MXM card on its own without paying through the ass for it will be a pain in the ass.
They know lots about this stuff over at notebookreview.com forums. Asus sometimes use MXM i think, Acer surprisingly enough usually use MXM, Dell and HP etc never use it and have their own non upgradeable system.
http://forum.notebookreview.com/acer/396320-acer-mxm-models-cards.html
Check the above thread out, thats just for Acer. I looked into this extensively about a year ago, decided to hell with this minefield and built myself a desktop instead
