I'm not a stock market analyst, but I play one on the internet:
Agreed, stock buyback is a good thing for Nintendo. It generally occurs when a company wants to stimulate interest in the stock price and/or they see the stock at a good value. Afterall, who better to determine whether a stock is over/under valued than the company itself, right? In this case, Nintendo stock has taken a hit recently, mostly due to overreation from investors on Sony's entrance into the handheld market.
However, less stock does not neccessarily mean stock will be more valuable (oh if only it were so easy), and this is a point on which expensing stock options hinges. When a company gives stock options, this increases the amount of stock out there, so your stock reflects a smaller percentage of the company, but the stock price has not neccassarily gone down in value.
While stock buyback is almost always good, sometimes it takes place when a company wishes to fortify itself in case of a takeover attempt. (not to fan any rumors as this is not the case)