Beyond the non-compete clause, there also is usually a "assignment-of-inventions" agreement. Which, to some degree, means if you are in a role where you might "invent" things, anything you think of while at the company is owned by the company whether you told them about it or not. The contracts can get complicated: if you have an idea completely unrelated to the company's business, and can prove you did not use any company or equipment or time, you might be ok.

Saying "I had the idea for Reader but I would not tell Google because they would ruin it, and I would instead quit and form my own company" is exactly what these agreements are aimed at.

http://www.contractstandards.com/document-checklists/inventi...