there is the Wildfire scalability project Pampero[/b][/url], you may want to look at it. You may use different DNS names for n servers and set the xmpp.domain of each server to foo.com. Doing this s2s will no longer work, sharing a MUC across two or more servers is also no possible, so you will have seperate servers and will not like this.
Doing it the other way, setting up one DNS record foo.com which resolves to im1.foo.com and im2.foo.com will also work in a very limited way. You may encounter heavy problems when adding users to a roster etc. as each Wifi server will use its own database.
So the easiest way which comes to my mind for a very quick fail over (all connections get dropped) is a mirrored LDAP if you are using LDAP, a shadow database and two Wifi servers while you use a layer-7 switch to make sure that only one Wifi server is used at a time.