I have the same error on a Centos 5.6 machine with the Vsphere 4.1 SDK. When i tried to check the status of my ESX Server with nagios i have that problem:

/usr/local/icinga/libexec/check_esx.pl -H 192.168.xxx.xxx -u xxxxxxxx -p xxxxxxxxxxxx -l cpu
CHECK_ESX.PL  CRITICAL - Server version unavailable at  'https://192.168.xxx.xxx:443/sdk/vimService.wsdl' at  /usr/lib/perl5/5.8.8/VMware/VICommon.pm line 545.

I check the Vsphere 4.1 SDK Documentation and i had already install all the requirments (Perl Modules, Openssl and libxml2).

I tried to re-install the XML::LibXML without results