I know as of this time custom objects are not available in forms. However, I do feel there will be a large use case for taking on multiple instances custom objects in forms.

For instance. If you are law firm and there is a case object and a client could have mulitple case objects which could include multiple fields associated with the "case" object being able to add multiple instances of the object that would then add to the custom object record would be ideal rather than multiple different submissions.

Example: Car lot as a form for a client who wants to sell their car. Car is an object. Client could want to sell multiple cars for a submission. Instead of creating 3 sets of fields for multiple car submissions, a user should be able to have one car object and a single set of fields for the car object and a submission should allow for multiple instances of the car object for a single submission.