The trick with a DSL "modem" is that it is actually a router.
Even today, to be able to route at a few MBps, you need a decent little processor in there. Plus, I think they consume a bit of juice because of that and the transmission. Dunno. It's an interesting question.
I don't think it would be easy (and it would be expensive) to put all that power in a lil PCMCIA card.
Maybe a type III card (two slots wide) but it may still stick out... might as well get a USB type.
Eric