Here's a function to do it which uses get-wmiobject and the -query
parameter:

######################################

function Create-ProcessWithWMI ($computer = ".", $commandline = $(throw
"Enter the command to execute.") )
{
    $ProcessClass = get-wmiobject -query "SELECT * FROM Meta_Class WHERE
__Class = 'Win32_Process'" -namespace "root\cimv2" -computername $computer
    $results = $ProcessClass.Create( $commandline )

    if ($results.ReturnValue -eq 0) { $results.ProcessID }  # Or just return
$true if you don't want the PID.
    else { $false ; throw "Failed to create process!" }

    Cheers,
        Jason