There was the usual gnashing of teeth on the Apple (AAPL) investor boards when the company’s share price dropped $6.18 ($1.8%) Monday, the day of Steve Jobs’ iCloud keynote.

Adding insult to injury, the stock continued to fall three out of the next four days. It closed Friday at $325.90, its lowest price since Dec. 2010, down 5.7% for the week.

But the teeth gnashers shouldn’t have been surprised. Apple’s shares have fallen the week of the company’s annual Worldwide Developers Conference every year since 2003. They fell 6.7% in 2008, the year Jobs looked so thin, and 9.1% in 2004, the year Apple previewed but did not release OS X 10.4 (Tiger). Average post WWDC loss: 5.4%.

Past performance is no guarantee, of course, but investors who sit tight may yet have the last laugh. The average gain in Apple’s share price from the close of one WWDC to the start of the next WWDC over the past eight years is 56.8%.

To put it another way, a share of Apple purchased before Steve Jobs’ 2003 keynote cost $19.30. Even after last week’s losses, it has grown in value 1,589%.

See chart below the fold.