Windows Clustering Services have been around for some time and most if not all critical servers in large enterprise are probably running on this technology. An important part of a cluster is the quorom disk which holds vital information to keep your cluster up and running. The two options in Windows 2003 for this quorum are shared disk quorum model where all nodes share the same quorum and the majority node set model where each node has a replicated copy of the quorum. The shared disk model is used most often because a lot of the clusters consist of two nodes. Do note that there is an update for Windows Server 2003 that enables a "File Share Witness" to create a majority node set cluster with just 2 nodes (KB921181 - steps required).
In Windows Server 2008 they have merged these two models which is now called Majority Quorum Model. Before Windows Server 2008 the quorum disk was a single point of failure. The risk was obviously low because quorum drives are usually installed on redundant storage devices but even highly redundant storage may fail sometime. In Windows Server 2008 however the quorum disk or as it is now called witness disk is no longer a single point of failure. Clustering now uses a 'vote' system where each node and the quorum device can be assigned a vote. A cluster remains online if just a single vote is lost meaning your cluster will continue to work even when the quorum is lost.
More information about failover clustering in Windows Server 2008 can be found here: