Pfsense bonding
Full bonding with pfSense
From version 2.0.1, it's possible to configure pfSense for policy-based routing and fallback over multiple WAN connections. One AAISP customer runs this with two AAISP ADSL lines, switching to an AAISP SIM card if both lines fail - see his blog entry for details (PDF Archive File:Multi-WAN + Multi-LAN + No-NAT routing with pfSense 2.0.1.pdf
However, pfSense does not yet have IPV6 support, and (as of 2.0.1) the recovery from fallback might not always work perfectly.