Craig Hart discovered a bug in the monitoring plugin/script check_esxi_hardware.py which occurred on self-built servers or generally on servers which did not have a CIM entry for Manufacturer.
The typical error message, when the script tries to handle the Manufacturer value, looks like this:
TypeError: cannot concatenate 'str' and 'NoneType' objects
The bugfix added in todays version (20120501) handles this case and sets the manufacturer string to Unknown Manufacturer.
Big thanks to Craig for his discovery and fix.