An issue exists when extracting Person or Group fields from SharePoint, specifically using SPSource but I'm guessing this issue transcends the use of SPSource due to the following article from http://support.microsoft.com/kb/934613.
The issue is that when you've extracted a field you may see an 'invalid' attribute value in UserSelectionMode="PeopleOnly". Basically this value should be 0 for people only as the schema defines this as an int. If you attempt to deploy a field definition in XML via Elements.xml you'll see the following error:
Feature definition with Id xxxxxxxx-xxxx-xxxx-xxxxxxxxxxxx failed validation, file 'ECM.SiteColumns\Columns.xml', line 17, character 98:
The 'UserSelectionMode' attribute is invalid - The value 'PeopleOnly' is invalid according to its datatype 'http://www.w3.org/2001/XMLSchema:int' - The string 'PeopleOnly' is not a valid Int32 value.
ECM.wsp: The Solution installation failed.
The value for UserSelectionMode should be set to 0 for PeopleOnly or 1 for PeopleAndGroups. I haven't tested yet, but I wonder whether this would be valid if the field was provisioned programmatically using AddFieldAsXml() method. This would then yield consistent behaviour with the <Customization> child element bug discussed on the SharePointDev wiki @ http://sharepointdevwiki.com/display/public/SharePoint+Custom+Fields?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+SharepointDevWiki+(SharePoint+Dev+Wiki+RSS+Feed)