Is it possible to manage 4.1 hosts with a vCenter server on 4.0?
What is involved in upgrading vCenter Server from 4.0 to 4.1 and what are caviats?
Check matrix to verify all mixed http://www.vmware.com/pdf/vsphere4/r40/vsp_compatibility_matrix.pdf
no, you cannot manage 4.1 Hosts with a 4.0 vCenter instance. The upgrade is pretty straight forward, although 4.1 is a 64bit instance so you may need to adjust your Host OS. Also, prior to the upgrade ensure you have a good backup of your vCenter DB instance.
No, you wont be able to mage ESXi/ESX 4.1 with vCenter 4.0
Go through http://kb.vmware.com/kb/1022104 and I am sure it would answer all your questions, if you still have them let us know
All the best !
Check matrix to verify all mixed http://www.vmware.com/pdf/vsphere4/r40/vsp_compatibility_matrix.pdf
Thanks for the information