(Get-SCOMClass -Name 'Microsoft.Windows.Computer'|Get-SCOMClassInstance)[0].'[Microsoft.Windows.Computer].PrincipalName'.Value
http://stevesbog.loungent.com/2014/02/19/using-square-brackets-in-powershell-queries/#.V7sELvmF5aQ
The simple trick I needed was to put the entire property in quotes.