Been doing something similar myself - you need to look at DCOM settings & WMI security, instructions are here:

DCOM: http://msdn.microsoft.com/en-us/library/aa393266.aspx

WMI: http://msdn.microsoft.com/en-us/library/aa393613.aspx

Interestingly, once I'd enabled DCOM for the relevant user, I could get the WMI data through PS despite a warning that I didn't have the necessary permissions!

Cheers

Gaz