I have published an Infopath form that creates a custom content type for a site. Using SharePoint Designer, I have created a workflow for the list that the content type is associated to. So why can I not access the fields of the Content type from the workflow?
InfoPath promoted columns are not treated like normal columns in SharePoint. They don't show up in Workflows, they don't show up for calculated columns, you can't use them for lookup columns, and they can't be edited in the SharePoint list forms, etc. In 2007 there was a special checkbox to promote the columns as read/write that could make them full columns, but I think they removed this in 2010.
I will check this tomorrow to see if there are any publishing options in 2010 to promote full columns.
Chris is correct, you cannot write to these columns as the form moves through your workflow as these columns are simply displaying the data from within the XML file. In order to work with the data and manipulate it, you would have to add custom extensions to write to nodes within the XML schema itself. However, you can use the data within workflows in ways like waiting for the field to change or as conditions to move through the process.