SPJeff

Here are the commands to change the global developer dashboard setting in SharePoint 2010 and 2013.

OnDemand Mode:

$sp2010 = [Microsoft.SharePoint.Administration.SPWebService]::ContentService.DeveloperDashboardSettings;
$sp2010.DisplayLevel = [Microsoft.SharePoint.Administration.SPDeveloperDashboardLevel]::OnDemand;
$sp2010.RequiredPermissions = 'ManageWeb';  #EmptyMask = all users.  ManageWeb = full control or higher.
$sp2010.TraceEnabled = $true;
$sp2010.Update();

On Mode:

$sp2010 = [Microsoft.SharePoint.Administration.SPWebService]::ContentService.DeveloperDashboardSettings;
$sp2010.DisplayLevel = [Microsoft.SharePoint.Administration.SPDeveloperDashboardLevel]::On;
$sp2010.RequiredPermissions = 'ManageWeb';  #EmptyMask = all users.  ManageWeb = full control or higher.
$sp2010.TraceEnabled = $true;
$sp2010.Update();

Off Mode:

$sp2010 = [Microsoft.SharePoint.Administration.SPWebService]::ContentService.DeveloperDashboardSettings;
$sp2010.DisplayLevel = [Microsoft.SharePoint.Administration.SPDeveloperDashboardLevel]::Off;
$sp2010.Update();

STSADM command line:

stsadm -o setproperty -pn developer-dashboard -pv ondemand 
stsadm -o setproperty -pn developer-dashboard -pv on 
stsadm -o setproperty -pn developer-dashboard -pv off