Interesting that they've moved to Lucene.net for search - I remember hearing them say that full text search is one of the reasons they were so happy with SQL Server.
"We rely heavily on full-text search on stackoverflow.com, which worked amazingly well for us under SQL Server 2005. Looks like that’s no longer the case for SQL Server 2008, unfortunately."
Here's the entry: http://blog.stackoverflow.com/2008/11/sql-2008-full-text-sea...