The NETGEAR ProSafe VPN Firewall FVS338 allows you to use an external serial dial-up modem as a fallback connection if your broadband goes down. I use this firewall because I'm on satellite and it falls back to dial-up automatically when I lose dish signal. You can also log into the router and manually disable the broadband connection, forcing it to dialup, so you can play online games which satellite can't handle.
But, from what I can tell, testing with my laptop connected to LAn via wire and having my laptop also connected via dial-up, it seems that Windows will automatically route internet traffic to the fastest connection. So I don't think it's possible to combine dial-up and broadband connections at once.
I've also seen higher end business class routers that combine two internet connections, but they only combine two broadband connections, and not broadband/dial-up.