Mike Laverick again shares another screenshot-filled pdf guide, this time with his upgrade notes. Overall it went OK, but he had a few issues with the VC upgrade. As always, you should backup, snapshot, and try it on non-production machines first.
The ESX upgrade part is relatively easy – given that the upgrade process can modify your configuration (as in my SSH example) and that moving a VM off an ESX host is simple. I think I would still prefer to wipe my ESX hosts and do a clean install – adding them back into VirtualCenter. This is not without consequences. It means losing some of that precious performance data collected over a number of weeks or months.