Not only can you write web services today that can be consumed by WCF (Indigo) apps in the future, but working with WSE 3.0 today to secure your web services will also help you get a handle on many of the concepts of WCF. I have written an article for DevSource called "Using WSE 3.0 Today to Secure Web Services Tomorrow" based on current guidance (based on WSE 3.0 and WCF Beta 1) on just such a topic.

I will also be doing a talk on this topic next week at DevConnections.

Thanks to Mark Fussell for his insights on this topic.