Right, it is something added to the IE user agent string so that the server can know whether the rich client (InfoPath.exe) is installed on the user's machine or not.

Vaguely related is InfoPath form templates running in a browser like IE or Firefox ... see http://blogs.msdn.com/tudort/archive/2005/12/13/503073.aspx for more info on that.

Cheers,
== Eric