Not sharing shares

Sun Microsystems [NASDAQ: JAVA] has started to buy back lots of it own shares. I got a proposition to sell my JAVA shares from them. Wonder why? The price is falling drastically as most other shares in the US lately. What's the reason to by back your own shares? Can it be that you have a lot of cash and when the stock price is going down it's easier to become a target for a hostile takeover. Having loads of "useless" stock shares instead of cash can prevent companies like IBM and HP to be to interested in Sun, or?