Microsoft Knowledge Base Article 812504 contains the following summary:
In Windows 2000 Server and the Windows Server 2003 family of products, the quorum disk must be a shared, block-level disk device that is accessible from all members of the server cluster. For a server cluster to be a supported configuration, the complete cluster configuration must appear on the Cluster Hardware Compatibility List (HCL). For additional information, click the following article number to view the article in the Microsoft Knowledge Base:
309395 The Microsoft support policy for server clusters, the Hardware Compatibility List, and the Windows Server Catalog
For Windows 2000, a prerequisite for qualification is that all components of the cluster solution are device qualified and cluster device qualified. The whole solution is listed on the Cluster HCL. For Windows Server 2003, a storage block qualification process is the prerequisite. Only device-qualified and cluster-device-qualified, block-level disk storage solutions can be used as disk resources in a server cluster.