A few years ago, Microsoft broke compatibility for Visual Basic. It wasn't a subtle break either, the difference between VB 6 and VB 7 made moving forward with existing code neigh impossible for all but the most trivial of applications.
While the upgrade story is still poor, Microsoft has been slowly chipping away at the problem. In their newest offering, Visual Basic 6 forms can now host .NET controls as if they were native to VB6.
One sore point for users of the first Interop Forms was the inability to create MDI child forms. Well, unfortunately that still is not possible. What developers can do, however, is create a child form in VB6 and build all of its contents in .NET. While this is not a perfect solution and will cause problems with things like merged menus, it does get developers a little bit further down the migration path.