I was bit happy after knowing that we can also add FieldAsXml in SharePoint but when I came to know something strange while using this method then that made me a puzzle ..
what basically this all about is :
when we add any field using this method then SharePoint just forgets to set Internal Name of the Field ,
Bil Simser has come up with a work around which makes things OK
I also heard somewhere that this issue is still there with SharePoint 2010 APIs ..
I would surely like to hands on it using SP2010 as soon as I touches SP 2010 (heavy Infrastructure ;)) environment