A form overwrites the default content type for a library when you publish an InfoPath 2007 form to update a SharePoint Server library that has multiple content types

When you publish a Microsoft Office InfoPath 2007 form to update a Microsoft Office SharePoint Server library that has multiple content types, the form that you publish overwrites the default content type for the library.

The default content type is the content type that appears first in the list of content types. This content type determines the type of content that the user opens when they click New on the SharePoint Server library toolbar. InfoPath 2007 publishes over the default content type even if the content type was not originally created for that form. If you want to update a different content type in the library, change that content type to the default before you publish from InfoPath 2007.

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.