When I run
Get-WmiObject win32_SystemEnclosure -Computer hostname | select serialnumber
it works for both local and remote hosts.
W
I just came to the exact same issue and found the answer here: http://powershellcommunity.org/Forums/tabid/54/aft/7537/Default.aspx
I had space characters at the end of each line the input file. If your file does too, just remove them and your script should work.