Setting up pfSense with multiple WAN connections configured for failover is easy. Setting one up in a pfSense that has Squid Proxy is not obvious - in fact, there is a lot of conflicting information. Ultimately http://forum.pfsense.org/index.php/topic,38882.0.html provided the answers I was looking for.
Interesting Note 27-Aug-2012: This configuration broke the ability of the package manager to check the repository and install packages resulting in unpredictable behavior at best. To compensate you must disable the Transparent Proxy and disable Allow Users on Interface as well as disabling the new floating rule that enables Squid to function.
Update 15-Oct-13: The proper floating firewall rule + adding your DNS servers to the Squid General configuration page will fix the broken updates. The Floating Firewall Rule you must create is: Pass, select your WAN and Opt1 (or whatever you called your public interfaces), direction: Out, protocol: TCP, Source: Any, Destination: Any, Destination Port Range: HTTP, then under Advanced, Gateway: select your failover group.