Be careful before you order that 3Com mini-PCI NIC/Modem card from Dell.
I ordered one the other day (I now have it), but it's been configured for the Latitude notebooks and has two special (i.e. ultra small) connectors on the side to make connections to the phone line (2 pins - should be simple) and (about) 12 lines for the network connection. I'm guessing that the 12 wires are for all sorts of things like the (4) ethernet lines, and the status LED's for link status, speed, collision etc.
Don't get me wrong though, the thing plugs in and gets found by Windows perfectly, just getting the signals out is going to be a pain.
The bad thing is that documentation with it sucks, there's no pinout or connector manufacturer information to start with. Whilst I'm sure many would just say "just solder some wires to it!", I'd rather get the job done right and make it look a little neater. I'll be scouring the DigiKey catalogue for suitable connectors.
Once I've figured that out, it should be easy to replace the double rear RJ11 phone jack ports with an RJ45 and an RJ11 on a new piece of perfboard and wire it all up, though I think I'll have to widen the rear panel a little as an RJ45 is a little wider.