Funny that you posed this question, ncisi74, as I have what some might now consider 'extensive experience' on the topic of using the Transverse Viking PCI ADSL2+ modem card with pfSense. The short answer is: if you tend toward being a masochist, go ahead and give it a try. The reasoning behind my advising you in this way is that there is a steep learning curve for getting the modem to work with your pfSense box, both hardware- and software-wise; the device is not cheap (ca. $135.00 with shipping); it has to be ordered from Europe; it will generate additional heat in your pfSense box; and, as Steve already mentioned above, it increases your risk of lightning damage to your pfSense box. Oh, and did I mention that the manufacturer of the modem has recently taken it out of production and no longer supports it via its web page? You might want to save yourself a lot of grief and stick with using the modem that your ISP has already supplied to you (in bridge mode).
All of the above being said, here is what you would need to do to realize such an elegant build (be prepared to waste a lot of time and tear your hair out in the process):
1. Buy the modem from its reseller in Belgium via the link Steve gave above (if it still has it in stock) and wait two to three weeks for it to arrive at your doorstep in the U.S.
2. Install the modem via a PCI slot in your pfSense box's motherboard--I had to use a special riser card (for my 1U Supermicro chassis and motherboard) and buy a special adapter cable to supply 12VDC to the riser card in order to make the modem card operational.
3. Configure the modem for pfSense--it should recognize its adapter without any problem.
4. Telnet to the modem to set it up in bridge mode and change its VCI/VPI settings to conform to your U.S. ISP's requirements (remember, the card was made in Australia where the standards are different--this is not an "optional" step).
5. Configure pfSense to allow access to the browser-based GUI of the card if you would like to view its stats in real-time operation (I finally accomplished this somewhat daunting task last night after otherwise using the card sucessfully for over nine months).
Good Luck (you will surely need it).