I would like to create a GUI extension for Experience Manager in Tridion 2011 to add a new button to the WorkflowDeckPage, any ideas on how to do it?
See if this helps, Bart has written a post on adding unanswered SO questions to his Dashboard. http://www.tridiondeveloper.com/dashboard-extension
I'm assume you are trying to extend WorkflowPanel. Currently there are no extension points in this panel (like ExtendableArea or extendable controls). So you can only create script on your own risk which dynamically over js adds new Button to WorkflowPanel. This script can be added to "Tridion.Web.UI.Editors.SiteEdit.Controls.WorkflowPanel" configuration group via groups extensibility mechanism.