It may be a little dated now, but the following article by Clayton Cobb gives an excellent explanation of how to embed SharePoint list data directly into your InfoPath form. Anyone who’s tried to do this has undoubtedly run into the problem he describes – the list items may pull into the repeating table correctly, but they don’t stay with the form once saved. If you open a submitted form later, it won’t have the same items as when it was submitted. This is because the items are only viewed in the form, not saved into the XML of the form like other fields are.
Clayton’s answer is a free plug-in by Qdabra that allows you to copy the list structure directly into the XML as a table. I have a “shopping cart” type of form solution I’m working on currently, and I’ll definitely be giving this a try.