I was surfing today and I noticed that some web sites of big companies where using .Net.
This doesn't mean that .Net is there main technology, but the extension is at least used in the URL...

McAfee

They use it in the FAQ's... here

Fabricom GTI (Suez)

At their main page: http://www.fabricom-gti.com

Dell

At their main site...  http://www.dell.com

Yahoo

Probably they are not really using .Net, but it is a .Net file on there domain anyway...
On this survey link: http://surveylink.yahoo.com/wix/p0473306.aspx