XCode has had a major redesign starting in version 4. Not only the UI and Compiler have changed, but many keyboard shortcuts did too, and for those used to develop using XCode 3 the transition is a bit hairy. While Apple does not provide a complete reference guide for all the shortcuts (none that I know), some handy developers out there do.
The XCode 4 Keyboard Shortcut Reference Guide has a very complete and extremely useful list of all shortcuts, organized in a pretty PDF file. The download links are: