As I have previously posted, I did a similar setup and it works well. I have not done a live failover, but have tested the replica.
I started with two IBM x3200 servers, call them A and B. They have redundant power supplies, 32GB RAM, both running Win2K12 Standard. I installed SBS2008 long ago for AD, SharePoint, and Exchange, so the first thing I did was to virtualize that machine on server B using HyperV. I decided to go with 840 Pros in RAID1 1 on server A, then made a virtual Server 2008 machine on server A and moved AC onto it. This is in response to my one and only AC support call, when they commented that they did not like to see AC running on the domain controller. At this point, the configuration was no better than before, but AC is isolated and both AC and SBS are virtualized. The 840 Pros made booting the AC machine lightning fast, as was response time to queries. So far seems reliable enough.
After everything was running correctly for a while, I made a replica of the AC machine and put it on server B. I set replication to hourly. So far it's been running in the background with no hitches. If Server A crashes, I will simply point the AC clients to the replica on Server B.
To answer the question of the domain controller going down, I made a secondary controller on another virtual machine on a stand-alone computer, not server grade.
