Agreed - I haven't seen any type of on-site "datacenter in a box".  What type of scale and length of eval are you looking at?  A few of our practices, in order to demo and evaluate VMware, built clusters with old servers backed by another server (running OpenFiler with a decent amount of DAS) for a cheap SAN.  While not ready for any type of enterprise load, it was definitely adequate to evaluate and understand enterprise functionality such as vMotion, provisioning, etc.

Heck...I did a technology update at on of our forums last year.  I was inspired by VMware's lap environment (running ESX on ESX) and ended up building a full vSphere 4 environment on a tricked out laptop (ESXi running on the laptop, hosting two more instances of ESXi and an OpenFiler VM for storage).  See http://www.vcritical.com/2009/05/vmware-esx-4-can-even-virtualize-itself/ for details.

Hope this helps!

Dave