I have fairly extensive knowledge of this. We recently migrated to Marketlive a tier 1 e-commerse provider and the integration is all done through XML both in and out.Look at the name of the forums.
MOM XML is a dumb flat file in xml format. Basically I mapped xml data using a great product called mapforce from altova. It has worked great for me. The learning curve was a little steep but it cost $1500 instead of 20-50K on paying someone to do the integration.
I am always happy to share what I know so please feel free to ask or msn or even call me.