Suppose I wanted to develop a mobile GUI application...
qhoxie: There are a number of different avenues you could take on this:iPhone - Obj C.  and xcode makes for a great development environment.  Problem is, xcode makes for the only development environment.  You will have to distribute it through the App Store, unless of course it is for jailbroken phones.Symbian - Common platform to many phones.  C++ or Java with fairly good SDKs.  No App Store equivalent could be a pro or a con.Android - Not many handhelds out yet.  Java with solid SDK and tools.  Central app repository for distribution.Blackberry - New central app repository.  Variety of development environments including VS.  Java for the most part.  Well documented SDK.Barring some really bad code, you should not have performance issues with any of these platforms.  Depending on the simplicity of your app, it could be worthwhile to deploy on a number of these.