Enable / Disable over stsadm:

stsadm –o getproperty –pn developer-dashboard get the current setting from developer dashboard (On |Off | OnDemand )
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stsadm –o setproperty –pn developer-dashboard –pv “On” set the setting from developer dashboard (On |Off | OnDemand)
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Enable / Disable over powershell

Turn On: Set-SPFarm –DeveloperDashboardEnabled

Turn Off: Set-SPFarm –DeveloperDashboardEnabled $false

 

Found this over @ http://blogs.technet.com/patrick_heyde/archive/2009/11/16/sharepoint-2010-enable-using-developer-dashboard.aspx