Managed to track down the below in the logs, definitely certificate related. Filed an SR since everything else indicates the certificate is OK.
The Webclient log file vsphere_client_virgo.log contains the following error
"[2013-06-18 09:15:30.168] ERROR [ERROR] http-bio-9443-exec-5 c.vmware.vum.client.adapters.updatemanager.aspects.LoggingAspect Exception caught in class 'com.vmware.vum.client.adapters.updatemanager.UpdateManagerService', line 160 com.vmware.vim.vmomi.client.exception.SslException: com.vmware.vim.vmomi.core.exception.CertificateValidationException: Server certificate chain not verified
[2013-06-18 09:15:30.184] ERROR [ERROR] http-bio-9443-exec-5 com.vmware.vise.data.query.impl.ServicePropertyProviderAdapter Error when fetching property {baselineComplianceStatus} from property provider {class com.vmware.vum.dataservice.impl.CompliancePropertyProviderAdapterImpl} com.vmware.vim.binding.vmodl.MethodFault: There was an error connecting to VMware vSphere Update Manager.