Marginal Revolution points us to a study by an American professor which blames umlauts for German grumpiness: "Hope College psychology professor David Myers says saying a vowel with an umlaut forces a speaker to turn down his mouth in a frown, and may induce the sadness associated with the facial expression." English, he claims, involves broad 'ah' and 'eh' sounds which require you to mimic smiling motions. This story was originally reported back in 2000 by the BBC.
Hat-tip to Marian Wirth, valued German Joys Commenter and freshly-minted blogger, for the link.
Now to the substance: I have my doubts. I'm not going to address Myers' theory in detail, because there's not enough information about it in the article to draw an informed conclusion. I have an email in to Myers to see if he published his results anywhere; I'd love to learn more about the methodology and conclusions.
However, I can't see how umlauts could be the culprit here. Some background for non-German speaking readers, umlauts are the two little dots on top of a, o, and u in the German language. They change both the pronunciation and the meaning of words considerably. 'A' in German is pronounced like the 'o' in 'gone,' while 'ä' is more like the 'a' in 'bake.' 'O' is pronounced a lot like the English 'o,' but 'ö' is pronounced like "er." Pronouncing 'u' with an umlaut is tricky, it's a sound halfway between 'u' and 'e' that non-native speakers almost never master.
What I don't understand about the theory is that rough equivalents of the umlauted sounds of 'a' and 'o' exist in English, so the umlauted 'u' is really the only way in which umlauts introduce a sound into German that doesn't exist in English. I have tried smiling while saying umlauted vowels, and it seems to work just fine, it's only a little tricky with the 'ü', since you've got to tighten your cheek muscles a bit to really get it right.
Mr. Wirth noted another potential objection: If Germans are so glum because they have a few umlauts, what about Finns and Turks, who decorate their vowels (and even their consonants) with an almost-ludicrous variety of diacritical marks?
As I said, I'll withhold an analysis until I get more details. But color me, so far, not yet convinced.