I just saw the word "goodbye" spelt as さよなら. Having clearly heard the long o sound before, I thought the word was misspelled. However, a quick online search shows both さよなら and さようなら being used. Is there any semantic or usage difference between the two words?
Here's how I'd describe the uses of さよなら:
Sometimes 1 and 2 can be expressed with さようなら, but it's less common. Note that it's always さよなら in baseball and in さよなら公演.
Keep in mind that さようなら is the older and more "proper" form of the word, and さよなら may not always be appropriate.