Currently the OpportunityProduct is not available to be the primary entity in a relationship despite MSDN documentation indicating that CanBePrimaryEntityInRelationship = TRUE. There does not appear to be any method (supported or unsupported) of creating a lookup to the OpportunityProduct (entity does not show in OOB Lookup Creation Dialog using Customization UI, XML modification fails on import, etc.).
There are cases where it would be advantageous from a business standpoint to add multiple Opportunity Products to an Opportunity with a reference to another Opportunity Product (i.e. selling 2 pieces of equipment and one service plan, you want to specify which piece of equipment the service plan is for, as an example). The documentation seems to support this model but the application seems to reject it.