Hi Dawn,
I don't believe there's a global field editor in Marketo like your wanting. Since you can change the pick-list values from form to form in the editor there wouldn't be a master.
Now I haven't tested this, but if you use Salesforce or another CRM, possibly changing the field in there will then reflect a change in your Marketo forms. Not sure if it's possible, but it's worth a try. If your pick-lists are customized and different than your CRM values, then I don't think this would work.