I installed Xcode 6.3 which includes support for Swift 1.2. It turned up a ton of error messages, which are mostly casting issues.
I navigated to the storyboard, and cannot go back to any other
I gather from the apple developer forums that this is an @IBDesignable issue. Especially in projects that use custom fonts, additional xibs, etc.
I have somehow fixed my issue by removing all @IBDesignable from swift UIView class definitions. You can open your project directory with TextMate or other, search and remove all "@IBDesignable"
However I still think this is a MAJOR bug, that needs to be worked on.. so keep filing bug reports to Apple.