At our small office we have two connections - a high-ish speed DSL and a slow cable - just in case one goes down. We have 10-ish people using the DSL line, and we switch to the cable when the DSL has problems (DSL is pretty new in our area, and the ISP is having teething problems).

Is there a way we can use both of these connections at the same time? Or rather, link both connections into one box, and have that box route requests to all other boxes?

Hardware-wise, we have a standard cable modem and for the DSL we have a Cisco 878. Both plug directly into the Wireless Access Point we use dependent on which one is active at the time.

just go to lartc.org and check out chapter 4, should be what you're after, assuming there is a linux router availabel at some point. i've said that an awful lot the last few years!

Buy a Linksys RV042. It will let you use two DSL/CABLE services with failover and load balancing.