I think there is some cisco magic you could do with 'dial backup'... you may even be able to rig this up with an ibgp session (even if that goes out over the external provider) to swing the routes.
NOTE: this could make your site oscillate if there are connectivity issues between the sites, it could get messy FAST, and it could be hard to troubleshoot. Basically look before you leap :)
This link may b e of assistance: http://tinyurl.com/l8zpm
This link asks me for a login...
to get greed into it.. are you sure you want to be 'stuck' with a single carrier? :) What if the carrier dies wouldn't you want redundant carrier links as well?
I'd love a multi-ISP solution. I just assumed that anything involving more than a single upstream AS across the two links would leave me having to consider BGP convergence instead of just IGP reconfig. I didn't presume that that would likely be something that happened in seconds. If there's a fast approach to be had here, I'd love to hear it. thanks, a.