I guess what your saying is in an error making the site entirely read only from another data center. That could be of some use but I would imagine that would require a complete rearchitecture to separate out all writes and also have a method of recombining stuff like logging which will still be written later on.
On the git level though it would be useful for a major use case of github which is to grab code.
My point was really from the standpoint of a system and network wide outage for which it will take multiple hours to recover. Its about business continuity. I've been making comments from the standpoint of a user and the open source community but what about from the other side. I guess its a subscription like service for paid for customers so it has no immediate consequence on revenue but in places I've worked downtime is money lost and customers would actually require us to have SLAs and disaster recovery procedures in place before they would sign contracts with us. Anyway, yea the step from single server to multi server, to multiple datacenters does normally involve some re-architecting at the state layer, everything else is load balancing.
My first recommendation would be setting up a warm-standby or even a hot-standby if possible in another datacenter. It all comes down to state, meaning the data/repos. They are currently using drbd for block level replication of disks in their file servers, this can be done over wan using something like drbd proxy. Another option is to write something in house which replicates write requests to git repos hosted elsewhere. They wrote a collection of services to access git repos on the file servers via rpc, its entirely plausible to extend this. If they are using mysql anywhere thats a pretty simple task for master-slave replication or semi-sync replication in mysql 5.5. Redis also supports replication.
I'm sort of hinting at high availability with read/write but starting out with a read-only solution hosted in another datacenter on the order of 60-120 secs out of sync is not impossible. I think we would all benefit from that kind of continuity right?
Replicate your data to a secondary site, keep it in read-only mode, set low ttl's in dns. Take it a step further and start looking at globally distributed file systems if you are feeling adventurous.