[NSObject : AnyObject]?’ does not have a member named ‘subscript’ error in Xcode 6 beta 6

As mentioned in the Xcode 6 beta 6 release notes, a large number of Foundation APIs have been audited for optional conformance.
These changes replace T! with either T? or T depending on whether the value can be null (or not) respectively.

notification.userInfo is now an optional dictionary:

class NSNotification : NSObject, NSCopying, NSCoding {
    // ...
    var userInfo: [NSObject : AnyObject]? { get }
    // ...
}

so you have to unwrap it. If you know that userInfo is not nil then
you can simply use a “forced unwrapping”:

var info = notification.userInfo!

but note that this will crash at runtime if userInfo is nil.

Otherwise better use an optional assignment:

if let info = notification.userInfo {
    var keyboardFrame: CGRect = (info[UIKeyboardFrameEndUserInfoKey] as! NSValue).CGRectValue()
} else {
    // no userInfo dictionary present
}