Poll: Using a Cable modem with SDSL

[shpshftr]

I have a AT&T cable modem and a Interaccess symmetrical 1.54mbs DSL. I'm trying to set them up so that if the DSL goes down the computer will switch to the cable modem automatically. That part is easy just by assigning multiple default gateways to the machine. The first is the address of the DSL router, and the second the address of the router for the cable modem. The problem is there's no way to switch the DNS servers so after switching to the cable modem it will still attempt to use the DSL's DNS server's rather than switching to the DNS servers for th cable modem. It works once it gets down the list of DNS servers but that causes page loads to take forever. Anyone have any ideas?