Linux-HA or (probably easier to set up)
DynDNS and a small shell-script
Linux-HA looks like it needs to be installed; wouldn't work on x10.
DynDNS is probably the only one that'd work on x10, assuming it doesn't require -any- work at all on x10's side. If it does, it probably won't work there either.
DynDNS isn't 100% foolproof though because some systems will store the DNS address it gets when it first goes to look for it - if I were to view the site, then the server dies, I'll start getting timeout errors until I clear my dns cache and it goes to get the new address. It's not 100% perfect, but it -might- help.
The other part of the issue is there's been times that the server and service are up, but taking a long time to respond - I'm not sure how DynDNS treats that for failover purposes...