I'm going to move another batch of databases between servers this weekend, so some customers will experience brief outages during the maintenance window. Again, downtime will vary with the size of the database, but should generally be under five minutes.
I'll also be moving FogBugz and Kiln On Demand to new load balancers. This will incur a very brief service outage (only a few seconds), but should improve performance and reliability across the board.