Actually you don't do the updates on the production server, you do them on the staging server (your shadow server), test test the site, then swap the two servers. Swapping can fail, so you never do it until the last minute. However, this is only really do able if you have the infrastructure, you will not be able to do this with a standard web host. Can do this with cloud hosting however, Amazon, Windows Azure (easy, seamless todo in Azure). But if you are wanting to do this on your own servers its going to cost a lot of pretty monies.