When running Update-Help using the -SourcePath parameter on a machine without an internet connection that has had the help updated already, the command fails with "Unable to connect to Help content. Make sure the server is available and then try the command again." The Save-Help and Update-Help combination works perfectly when updating the help on a system with no help installed. I've seen this on Windows 7 and 8 (both 64-bit).
Deleting *helpinfo.xml files from the PowerShell folder ('dir $PSHome -filter *helpinfo.xml -recurse | Remove-Item -Confirm') seems to be a workaround for the issue.