Now that interest in WPF is really heating up, we've been getting more inquiries about WinForms and WPF interop. Fortunately, we have a lot of coverage in the SDK docs, which shipped in November:
Migration and Interoperability
Also, we have lots of sample code:
We have some designer-oriented topics as well, which shipped in Orcas Beta 1. These haven't been published at MSDN yet:
Hope you can use them to build some neat hybrid apps.