I've just released Fault Tolerant Router, a tool to manage the status and generate config for multiple internet uplinks. It's quite well described on the project page, I needn't repeat it here:
https://github.com/drsound/fault_tolerant_router
The tool is written in Ruby and not OpenWrt specific, but maybe an OpenWrt package will follow. Meanwhile I would like somebody to test it on OpenWrt. Thank you.