Executing Powershell SMO against a cluster with elevated privileges to update service accounts
I ran into the same problem, looks like its an SMO bug (1st link) (however I have not tried the CredSSP stuff from the second link below)

http://social.technet.microsoft.com/Forums/en-US/winserverpowershell/thread/265ee700-182d-4a99-96f8-cada117fd3d3

http://www.ravichaganti.com/blog/?p=1230

Please let me know if that works..