Check out the Microsoft InteropForms Toolkit 1.0

This toolkit helps you bring the power of .NET to your existing VB6 applications, by allowing them to display .NET WinForms from within the same application. Instead of upgrading the entire code base, these applications can now be extended one form at a time. The goal is a phased upgrade, with production releases at the end of each iteration containing both VB6 and VB.NET forms running in the same VB6 .exe process.

Screencast available here: