I'm not sure what switch you are referring to, they've been using HAProxy and Lucene for quite some time.

Also, what would a No SQL data store give them (for central data storage - not caching) that they aren't already handling with SQL? Keep in mind that they also have relatively easily partionable data (if necessary they can break off any StackExchange site and database to their own dedicated hardware).

redis is nosql, and they are using it on linux

Raven DB (http://ravendb.net/) is a pretty good NoSQL option for the .NET/Windows platform.

Wow, I had no idea this project existed. Thanks for the link!

I was looking for something like this a while ago for my startup but we ended up going with Azure and using their Table Storage.

AppFabric Server is closer to Memcached than Redis. However since their persistence layer is SQL Server, they could have used AppFabric in place of Redis.

I'm guessing they didn't want to use a technology that is so recently out of beta perhaps?