As previously written on November 8th, Dell's Open Manage Offline Bundle didn't work with ESXi 5.0. Symptoms were that not all hardware information was shown in vSphere client and that the Nagios plugin check_esxi_hardware didn't work at all.

Now this issue is solved. Dell released a new version of the Offline Bundle for ESXi 5.0 (6.5.0-542970) which was installed and tested by fellow check_esxi_hardware plugin users Pascal Saul and Dave Stevens. The plugin works now fine again with this release. Dell also upgraded their tech note on this issue by now using the following words:

Dell OpenManage Server Administrator agent officially supports vSphere v5.0 / ESXi 5 as of December 2011, starting with OMSA 6.5 A02.

The new Offline Bundle can be downloaded from the following URL (as of right now, the file couldn't be found on the Dell download website):
ftp://ftp.dell.com/FOLDER00197341M/1/OM-SrvAdmin-Dell-Web-6.5.0-542907.VIB-ESX50i_A02.zip

Update May 11th 2012:
A new version (7.0.0) for ESXi 5.0 is now available:
http://www.dell.com/support/drivers/us/en/555/DriverDetails/DriverFileFormats?DriverId=VPTVV&FileId=2922404090&productCode=poweredge-r720&urlProductCode=False

In general always make sure you're using the newest VIB Offline Bundle for your ESXi (3.x, 4.x, 5.x) version.