The point is that the underlying software is now getting treated as a commoditiy rather than as one of the primary reasons for buying an offered hardware platform. Dell is not advertising "Buy our VMWare-installed server" (actually Dell is currently offering both Citrix and VMWare, not just VMWare) but rather "Buy our Virtual Server platform". Dell has decided that the emerging mid-sized company market for virtual machines is drawn more by purchase price than the brand of VM software. Check Dell's general spec compare page and you'll find a reference to a generic "Virtualization Hypervisor" with no mention of a particular vendor:
http://www.dell.com/content/products/features.aspx/virtualization_servers

Dell will (attempt to) offer anything it thinks customers will buy as long as the cost/benefit to Dell means profit. Dell's current advertising strategy appears to show that Dell is already positioning itself to easily toss aside (or, at least, de-emphasize) any VM software vendor in favor of any other that may offer better profit to Dell.

VMWare beware.