I am working on a project now moving 700 users to Exchange Online. I am implementing single sign on via an ADFS Farm and DirSync.
Not every organisation has load balancers or the even the ability to load balance external DNS.
A very important factor that needs to be taken into consideration is this. If the Microsoft Online Services federation gateway cannot communicate with a customers on premise active directory then users are not getting their email , lync or sharepoint.
Email will still flow, but the users will think the service is down. So because of this it is always a good idea to have some high availability with the ADFS farm.
So I came across a company called Cloudfloor DNS .Cloudfloor provide the ability to loadbalance the traffic via round robin or active passive to my adfs proxy servers.It is quite a simple service but a lifesaver in the event of one site going down and all users not being able to use their Microsoft online services.
Cloudfloor can also do geographical dns based rules. This could be very useful in a gloabl lync or exchange deployment. Australian traffic goes to australian exchange servers and lync servers etc.... etc...
To set it up takes 5 minutes and works perfectly when tested.