if thats the case you have lot of points to convince your higher ups to get vSphere upgraded.
VMware introduced truly useful features in 5.1 and they have enhanced the existing features. Also the headaches liked Storage APDs are handled very well in vSphere 5.1
Refere the follwing "Whats new" links. especially the storage one.
http://www.vmware.com/files/pdf/products/vsphere/vmware-what-is-new-vsphere51.pdf