A lot of customers I've been talking to are really starting to recognize that the upgrade to vSphere is not just a simple version upgrade.  It's really something to be evaluated and considered before making the decision.  I saw this blog article that gives some good things to consider relative to an upgrade.

http://itknowledgeexchange.techtarget.com/virtualization-pro/should-you-upgrade-to-vmware-vsphere-4/