LARTC - Apr 2006 - multiple gateways problem

I have the following situation:

I have a linux box (not in router-mode) with 2 NICs and 2 gateways. I 
want to be able to switch gateways
when one of them is "down". The tricky part is that, by
"down" I dont
mean an unreachable gateway, but a gateway
that is reacable but for some reason cannot route packets.

any pointers would be appreciated