S&P ratings do affect how Google will run the business: If Google's stock takes a hit, employees get restless and are more likely to look elsewhere for employment. Employees aren't always long term focused in stock prices. Google could use this as an excuse to buy back their own shares to drive up value.
Theoretically I guess Google could care about their S&P credit rating (as opposed to this stock rating) if it made it harder for them to sell them in the future. Judging from how low the rate they had to pay on those bonds I suspect they wouldn't have any trouble selling them anyway.
Their credit rating is very unlikely to drop while they are highly profitable and sitting on a pile of cash though.
[1] http://www.reuters.com/article/2011/05/20/us-markets-credit-...