The recently released Microsoft Visual Studio 2010 does not support mobile development for OS versions prior to the new Windows Phone 7. If you want to write software for Windows Mobile 6.5 or lower, you need to stick to Visual Studio 2005. So much for backward compatibility.
So, if you are happy to try the new waters of Windows Phone 7, you can download the relevant SDK from Windows Phone Development web site. However, if you are getting an error message, saying "WE’RE SORRY. AN ERROR HAS OCCURRED.", chances are you've hit a bug with the Microsoft's web site. If you are connecting to it from outside the US, you can try to access the US version once. Just follow this link, it should register you for the US version and all subsequent entries should be fine.