To enable FT for a VM:
Get-VM X | Get-View | % { $_.CreateSecondaryVM($null) }
This will choose a host, if you want to specify a specific host, pass the host's MOR in place of $null.
To disable, run:
Get-VM X | Select -First 1 | Get-View | % { $_.TurnOffFaultToleranceForVM() }
Note that in PowerCLI 4.0 Get-VM will return a fault tolerant VM twice, so we select the first one.