So I'm looking at this diagram: http://doc.pfsense.org/index.php/Configuring_pfSense_Hardware_Redundancy_(CARP)
Does this assume that the "DSL Router" at the top of the drawing has two physical RJ45 jacks, or a single one that you're splitting via a switch?
i.e. when I walk into the data center with 2 pfSense boxes, should I be expecting them to give me two separate RJ45 jacks to plug into, or a single one that I'm going to throw a switch in front of the 2 pfSense boxes for?
In the referenced diagram, you should only have one RJ11 for a DSL connection. The authors of the diagram have assumed that the DSL router has two physical ethernet RJ45 ports which are on the same subnet.
For a typical DSL modem that has a single ethernet LAN connection, you need to have the modem connected to an ethernet switch, which would also host your pfSense FW connections.
The way that you configure your pfSense IP addresses depends on whether you have a routed or bridged-mode configuration on the DSL modem.