I love the strips where you make fun of weird English words :D. This one made me giggle uncontrollably.
It's from the movie "10 Things I hate About You" which is a modern send-up of taming of the shrew. It was Heath Ledgers first movie actually, great flick. I highly recommend it. But it is, verbatim, from the movie.
And yep...you guys were right, that same joke setup did appear in that flick. I just did a Google search, and it came up on IMDB (http://www.imdb.com/title/tt0147800/quotes):
Chastity: I know you can be overwhelmed, and you can be underwhelmed, but can you ever just be whelmed?
Bianca: I think you can in Europe.
At least I can feel better knowing that the punchlines were totally different. :)