this should help
See also the official guide:
http://www.vmware.com/pdf/vsphere4/r41/vsp_41_upgrade_guide.pdf
IMHO, If you have vCenter you can upgrade it first and then use VUM to upgrade the hosts.
PS: the update 1 of 4.1 is now available.
Andre
I just used VMware Update Manager (VUM) from within vCenter.
It took care of everything and I had no issues.
When I did my upgrade, wanted to use VUM but the tech warned me that (at the time ) Update Manager had concerns. I took the path of copying the upgrade file to each host and ran it manually.
I had no problems doing each host this way.
Good luck.