One of the more successful traders I've heard of trades nothing but WMT. Although my understanding is he combines trading the options along with trading the stock so I guess you could say he's trading one company.
If you're talking about daytrading one stock, you're better off being more versatile. There are certain scenarios which a trader can miss out on if he/she chooses to limit themselves to a single stock. IE. triple witching, economic #s, news(!)...