According to this article, Windows Phone 7 is to be launched on October 11th, and the first devices will be available in Europe on October 21st.
However, you can already get the development tools from Microsoft Windows Phone Developer website. Unfortunately, you will need at least Vista or Windows 7 to install Visual Studio 2010 Express for Windows Phone. So if you run an XP, you will need to upgrade.
You can also play around with the Silverlight for Windows Phone toolkit which according to Tim Heuer is really bringing a lot for the developers, making their life easier by providing missing controls and consistency.