I use v3.3.2 with PHP5, and while assembling a message it says:
And indeed: I can't find where Message.php would load the Swift class, though it is being referenced on line 85 (or more obviously on line 87).Fatal error: Class 'Swift' not found in <my_Swift_root_path>\Swift\Message.php on line 85
Only include_once()-ing Message.php has been sufficient in the past, so I suppose this change was not intentional.
I suggest everyone who has this same problem to circumvent it by including Swift.php right before Message.php!