hello mburguk
you cannot have two default routes in PIX. so there is no way you can do failover between ISP's when the routers are connected on two different interfaces of PIX...
alternately, you can have both the links terminated on the same router and connected to one of the interfaces of PIX. you can put a default route from the PIX to this router . you can configure nat/static/global for different inside subnets to the corresponding IP address range of the ISP's. but you cannot automatically failover when one ISP goes down. you will have to manually change the configurations on the PIX.
you should allow the router to do this. configure BGP and/or OER for multihoming between ISP's . automatic failover, traffic load sharing etc can happen through this.
hope this helps.. all the best...
Raj