In defense of my edit to add Transformers to the list of film examples, I can think of no more authoritative source to state that a particular item is a MacGuffin than Roger Ebert.  As someone else noted in the revert of my edit, Roger Ebert does not rely on Wikipedia to make this determination - he relies on his professional judgement and merely directs those who don't know what a MacGuffin is to this page.  Great press for Wikipedia!  I'll admit that I thought it would be clever for those so directed to find the Allspark in Transformers specifically listed as a Macguffin, but that does not detract from its timeliness and relevance as an example.