Back in September, Microsoft's VB team gave developers a way to include .NET forms in VB6 applications. In their latest CTP, they also bring you the ability to include .NET controls on a VB6 form.
Probably the most often cited problem with the Interop Forms Toolkit was its inability to create MDI child forms. By using this toolkit in conjunction with otherwise empty VB 6 forms, MDI children can be created.
This CTP is in a very early stage and can only be obtained from the VB Team's web log.