With the availability of vSphere 5.0 HP published a customized ESXi installation ISO for HP servers.
There have been reports that this build includes an annoying bug: HP has included a license file that has wrong permissions set. That potentially causes errors once you want to assign an own license to the host.
You can fix this by removing the offending license file with the following commands (to be executed on the ESXi host after directly after it has been installed):
esxcli software vib remove -n hp-esx-license --no-live-install
and reboot the host. The command will remove the HP license and restore the original state of the host being in evaluation mode.
HP has also published an advisory describing the problem and providing a way to update their license package to fix the problem.