I saw this one on the VI Toolkit forums today and had to pass it on.  Someone who goes by ade.orimalade@e-mis.com writes:

$cluster = "< >" # Enter the required cluster name

$installerArgs = ‘REBOOT="ReallySuppress"’

Get-Cluster -Name $cluster | Get-VM | % { (Get-View $_).UpgradeTools_Task($installerArgs) }

So the key is to provide that REBOOT=”ReallySuppress” line to the UpgradeTools_Task() method.

Update!

So this first one didn’t work as was noted in the comments.  I didn’t have time to attack the problem myself, but as always, the VI Toolkit forum comes through.  Someone who goes by the nick jp1960 submitted this solution:

   1: $insParm = '/s /v"/qn /norestart"' 

   2: $updList = get-cluster -name $YouClusterNameHere | get-vm |

   3:     where-object {$_.powerstate -eq "PoweredON"} | 

   4:         foreach-object { get-view $_.ID } |

   5:             where { $_.guest.toolsstatus -match "toolsOld" } 

   6: foreach ($uVM in $updList) 

   7: { 

   8:     $uVM.name 

   9:     $uVM.UpgradeTools_Task($insParm) 

  10:     #Wait 30 seconds before starting another update task 

  11:     Start-sleep -s 30 

  12: }