You can do it this way - http://samsharepoint.wordpress.com/2013/04/30/infopath-get-the-current-user-without-writing-code/
As far as I know we all pretty much used it from this site - http://blogs.microsoft.co.il/blogs/itaysk/archive/2007/04/05/InfoPath-_2D00_-Get-the-current-user-without-writing-code.aspx . Yet it seems to be down missing. If you use wayback when you'll be able to view a copy I suspect.
http://web.archive.org/web/20120303171712/http://blogs.microsoft.co.il/blogs/itaysk/archive/2007/04/05/InfoPath-_2D00_-Get-the-current-user-without-writing-code.aspx