N. Virginia in my experience is by far the least reliable region on EC2/EBS... Fortunately our app servers are across 2 zone in the region... but our db server is just a lone master... Our slave is down... Very nervous.
I run a master with a "hot" master each in a different AZ and slaves of each in their respective AZs for days like today. Expensive, but makes it easy to sleep at night.
The slaves have their EBS disks snapshotted every 30 minutes, the master every 24 hours.
A "full-spec" machine, another X-Large with 4 EBS volumes that I can fail over to. Its in circular replication with the other "active" master (only one is receiving writes at a time). These instances are only snapshotted once a day to keep them as fast as possible.
N. Virginia is by far the most used region. There's more stuff to fail and any failure will affect more people. I don't think there's anything inherently less reliable about the geographical location of the building.
The slaves have their EBS disks snapshotted every 30 minutes, the master every 24 hours.