At yesterday’s VMware event, I wandered over to the Veeam stand to get myself a demo of their nworks Management Pack for VMware. This allows OpsMgr (System Center Operations Manager 2007) to natively monitor ESX(i) without installing an agent on the host, with or without vSphere. They fired up an RDP session and gave me a guided tour. I was impressed with the solution. It had all you would expect from an OpsMgr monitored object: alerts, knowledge base, diagrams, and reports.
This reinforces the fact that even if you do deploy ESXi then this does not rule out the use of what I believe to be the best monitoring solution out there (even if it is my job to convince you of that!). With the nworks management pack and OpsMgr, you can include the mainframe-important-like virtualisation layer in the management of your hardware, operating system, services, application, and SLA stack.
BTW, if you are an SME then you can also use this management pack with System Center Essentials.