Microsoft announced yesterday that the VS Express products will be free forever. I know there had been some confusion over this (and whether you'd have to pay to download new copies in December). This announcement hopefully clears all that up. You will not have to pay anything ever, and they will continue to be free next year and beyond.
I've also seen a few people ask questions about whether you can build commercial applications with the express editions. The answer is definitely yes -- there is no license restriction on the apps you build (or sell) with Visual Web Developer or any of the other Express products (this also includes SQL Express -- which fully supports commercial deployments for free as well).
If you haven't checked it out yet, I'd also recommend visiting the Coding4Fun website. It has a number of great articles and starter kits to download that you can use with the Express products (as well as the full VS). The Lego Mindstorms Stater Kit looks especially cool:
I was a big Lego fan growing up. I'm looking forward to playing with them again using .NET! :-)
Hope this helps,
P.S. One amazing statistic -- we've had more than 5 million downloads of the VS Express products since their launch 5 months ago. Wow.