I want to provide redundant Internet connectivity to a home network. I
have dsl AND a cablemodem and an account with dyndns.org. When
internet on the cablemodem goes out I need it to update the dns so it
points to the dsl's ip address.

How do i set it up so that if one ISP fails it switches to the other automatically?

Is there an existing hardware solution that I can implement?

you can write a simple shell script to do this. 

if then statement. 
or an expect script. 

(Linux)