There are people who play credit card shuffle to maximize the value out of points / cash back / bonus, with balance transfers, etc. Their money making comes from those adders, not from investing borrowings.
CC companies have gotten wise to this and many (most?) charge a percentage fee for balance xfers. It would be pretty hard to borrow from a CC for investment and keep a sustained balance going over a long period at interest rates that would be favorable.
Also, credit rating and history dictates the amount of credit to be extended. Might be hard to get this to scale to make substantial return. Probably nothing compared to IMing, as Defon points out.
Then there is the problem of getting the cash from the card to invest in the first place. ATM withdrawals/cheque writing (or similar) might be exempt from the intro rates, plus there may be a transaction fee along with a cap on the amount withdrawn.
As Defon rightly suggests, could use the CC to purchase products to sell, but that is an entirely different scheme and risk profile.