Pardot custom fields previously could only be mapped to salesforce.com custom fields. The default fields (first name, last name, etc.) were reserved for Pardot default fields. Unfortunately that left several unused fields that could not be mapped (status, mobile phone, etc.).
Now you can create custom fields for these and map them to appropriate salesforce.com field by selecting the correct API name from the drop down. Note that you may need to click the green refresh icon for to populate your list of available salesforce.com fields.
I have attached a screen shot for reference.