How do you monitor your HP hardware while running virtual servers?

You should be using the HP-specific builds of VMWare ESXi or adding the HP offline bundle to your existing ESXi installations. The full set of drivers/utilities for your generation of server and VMWare version are here.

The main thing you'll need is the HP offline bundle, which will provide a few more health agents than you have now, and will include disk and array status (ESXi does not provide this by default).

The other helpful package is the HP offline utilities package, which gives you hpacucli (CLI array management) and hponcfg (CLI ILO management).

From there, you can set standard alerts in the vCenter setup under the "Hardware Health" category. It's a nice umbrella, since power supply, fan status, temperature, disk and even ILO status will report. vCenter uses SNMP traps by default, but it is easy to also add email alerts.

See:
Configuring vCenter alarm for loss of psu redudancy
and
How to find out more hardware details using only ESXi4.1 and (possibly) shell

HP System Insight Manager support also WMware Esxi managed systems as reported in the support matrix http://bizsupport1.austin.hp.com/bc/docs/support/SupportManual/c03341309/c03341309.pdf If you like SIM you can continue to use it.