One of the new features of vSphere 4.1 is called Storage IO Control (SIOC). SIOC is aimed at alleviating one of the most prevalent issues with hardware virtualization today - storage. Scott Drummonds does a good job defining SIOC in a recent article. Scott states "...the goal of SIOC is to identify this trend at the VMFS volume level and take corrective action to protect high priority virtual machines. The fundamental change provided by SIOC is volume-wide resource management. With vSphere 4 and earlier versions of VMware virtualization, storage resource management is performed at the server level. This means a virtual machine on its own ESX server gets full access to the device queue. The result is unfettered access to the storage bandwidth regardless of resource settings. SIOC will throttle virtual machine throughput once a volume's normalized latency crosses a threshold. The throughput is limited by decreasing each virtual machine's access to the queue to an amount defined by its relative shares. The following figure shows this in action."
There is a video showcasing SIOC as well: