Back in 2010 I wrote a post about how to get the iSCSI iqn of an ESXi 4.x server using vSphere CLI from the vMA or any other system with the vCLI installed on it.
The method described in that article is still valid for ESXi 5.0 since the old vicfg and esxcfg commands are still available, however with 5.0 version you can get a similar result using the new esxcli namespaces, following is how to do it.
First task is to get a list of the iSCSI HBAs in order to know the name of the software iSCSI initiator.
Next we get the info of the adapter.
Look at the Name field to get the iqn and we are done.