It is more like a work-around for RFC2616, the HTTP 1.1 spec, which says that compliant clients SHOULD NOT make more than 2 simultaneous connections to one server. This interacts rather poorly with modern sites, which may well need to load 50+ files on a single page view.

Much of the YSlow recommendations center on various ways of getting around this limitation -- for example, using multiple redundant domains (images1.example.org, images2.example.org... permitting you to do 2*N downloads in parallel instead of 2), putting all your CSS/JS in one file, image spriting, etc etc.

As far as I can tell IIS supports pipelining just fine since version 6 (2003). See http://en.wikipedia.org/wiki/HTTP_pipelining