I'm looking for a sample on populating a secondary data source with the list names from a SharePoint site.
This article has a brief description on how to add choices from a SP list to a dropdown control: http://office.microsoft.com/en-us/infopath-help/insert-a-drop-down-list-box-HA010103020.aspx
Basically, you need to add a receive connection to your SP list. Then in the dropdown properties, set it to get choices from an external data source and select the fields to use as the display name and values. Hope this helps!