I was more than surprised to find that Visual Studio 2010 (beta 1) has moved from "Any CPU" as default platform target to "x86". In short, every application you create with Visual Studio 2010, will not run on 64-bit Windows without compatibility layer (WOW64).
While this may seem like a minor change, I consider it a major problem in 64-bit programming. When Visual Studio 2008 defaulted to Any CPU, that did caused whole range of bugs and errors. However, by resolving those errors, developer was at least somewhat introduced to problems with his code in 64-bit environment. If he could not resolve it, he would switch it manually to 32-bit.
This change just seems like allowing careless developers to stay careless. In short time, this will not cause too much problems, but in long run you will get bunch of code that requires 32-bit compatibility layer (32-bit Windows will not be with us forever).