I thought about adding a 3G Module to my MBP17 as well.
Basically, all you need is a USB-Port and any USB-based 3G stick that you can disassemble and somehow connect to your MBP.
The EC34-Slot privides a USB-Port. In fact, virtually all 3G cards, most memory cards for EC and most card readers for the expresscard slot use this USB port and NOT the as well provided PCIe interface.
There are adaptors for ExpressCard-Slots which expose this additional USB-Port or which even act as an USB hub to provide 3 or more additional USB ports. Virtually all 3G-Cards for the ExpressCard-Slot use the USB-Part, not the PCIe-Part, in fact.
Then there are "developer cards", exposing the EC34-included USB as well, some even have space for soldering stuff to it.
There are two things these adaptors have in common:
1.) You can't really buy em. I tried to find them on some EU stores, but there was no Store (online nor offline) which would provide them.
2.) They move the USB connection to the outside of the MBP, which is not what I want.
Anyway, if it is possible to make a connection to the internal USB Port of the EC34-Slot, you might disassemble one of those cheap 3G-USB-Sticks (e.g. iCon 210 and similiar) and stuff the plain board + SIM into your EC34-Slot.
Added some creative work of making a nice housing for this setup, you might end up with a 3G Card that lives entirely inside your EC34-Slot.
I don't know how long the antenna has to be, but maybe it's feasible to make a case that has an antenna just at the outside of the EC34-Slot. The case should be made of epoxy or something that doesnt work as a shield to the antenna. The antenna of the iPhone or iPad is as well just exposed through a small portion of plastic, I guess this would work for a "3G-ified MBP17" as well.