When I read it earlier I was thinking "Oh, there goes Apple's stock price" until I read that he's staying on as chairman.
This is an obvious move so that when he eventually dies (we all do, except me, I'm gonna live forever) their stock doesn't go down the toilet. If you ever bother to watch Apple's stock you'll notice there is a serious point loss every time he gets seriously ill or rumors of his retirement.
It has already dropped...
The company released a statement announcing Jobs's resignation at about 3:30pm Pacific, after the markets had closed in New York, and as of 5pm Pacific, Apple's share price had fallen from 376.18 to 356.61, a drop of 5.20 per cent.
http://www.theregister.co.uk/2011/08/25/apple_stock_price_drops_in_wake_of_jobs_resignation/