I've got good news and I've got bad news...
The good news is: What you want to do is documented in Cisco's white paper "Enabling Enterprise Multihoming with Cisco IOS Network Address Translation (NAT)", available at http://www.cisco.com/warp/public/cc/pd/iosw/ioft/ionetn/tech/emios_wp.htm
The bad news is: Implementing it won't really do what you want because your settings on DNS cache lifetime will be ignored by large numbers of clients, delaying recovery from whatever you set the lifetime to be (typically 5 minutes) to anywhere from one hour to a full day.
You are also assuming that whenever one DSL line goes down, the other will stay up. Unless your two phone lines are physically diverse, which is extremely rare, this is not a safe assumption.
The cheapest way to get high availability for a web server (and it ain't that cheap, but much cheaper than doing it yourself) is to outsource the job to someone who specializes in providing non-stop web servers from a multiply and diversely connected facility with standby power and all that goes with it.
Even a cut-rate provider like Interland will provide better long-term up-time than your solution, and for about 1/3 of the monthly cost of your second DSL line.
If you want a fighting chance, consider replacing one of the DSL lines with a cable modem (unless both are strung from the same poles) or broadband wireless (if available). Don't forget a UPS and backup generator too, power is just as important as communications, and often even less reliable.
Good luck and have fun!
Vincent C Jones
www.networkingunlimited.com
