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 