You can do it with either using DNS or a firewall/natd that can redirect the traffic once the first server is down.
Using DNS you would keep the zone TTL very low and that would allow you to change the zone info and have it proprogate much faster then normal. Down side to that is that anyone using a dnscache will still hit the old down site.
With a firewall infront of your server you can create a 'rdr' type rule that will redirect traffic bound for that server to a seperate host and you can have the temp pages displayed from there. This would be a much better solution since you can easly turn it off and on unlike the DNS.
And when you say server down I am assuming you mean the box is completely dead in the water with no services running. Not just apache is down.
If you can afford it. I would have 2 identical web servers running and then put a OpenBSD/FreeBSD PF firewall infront of them and setup CARP+pfsync this will allow you to have automatic failover support and load balancing for all of your services.
Try 300 (5mins) and your DNS servers will feel it if you host alot of domains or see alot of traffic. Using DNS to do this is not the best method to achieve what he is looking for but it will work for smaller sites.