Cisco and WWW server

In short, no. You would either need to configure some sort of failover using software on the boxes themselves, or more appropriately, use a load balancer to monitor the individual IPs and balance the traffic based on some sort of rule-set (every other request, based on load, or only use 2nd in event of failure). A cisco router simply routes traffic from one network to another, and has no ability to re-address a packet to a different IP when its original destination IP is not responding on the network, which is what you are asking for.

OK this probably isn't the best way of achieving what you want and it may not work, however...have a look at Enhanced Object Tracking. For this idea to work you'll need to be able to create loopback addresses on your webservers and enable routing on each server. NAT port 80 to the loopback and make sure each webserver is listening on its loopback. Then create routes on the Cisco to each loopback via the ethernet interfaces of the webservers, and track the IP of the ethernet interfaces. Et voila, when the main webservers ethernet isn't pingable, the Cisco swaps the route to the backup web server.

Have a look at the "Not-so-Very-Static Routes" section:

Doesn't do much good if just the web-service stops, would it? Sounds like a pretty interesting, cost-effective solution, but not so sure I would jump into that with any kind of cash-flow at stake.

The OP asked for something based on ping. He could try an HTTP SLA monitor if it's the service rather than the server he wants to monitor. Presumably if there was any kind of cash-flow at stake, he would be paying someone to do it rather than asking on Usenet.