I'm surprised the reactions to this are so negative. Icahn may or may not be a good guy, but what does that have to do with his idea? I've thought for a long time that they should buy back more. They have a HUGE pile of cash, the stock is undervalued, and it shows faith in the company and its direction. $150b might be a bit much, but I don't see where the idea is bad.
Because Icahn doesn't care about Apple being undervalued. He cares about the money he could make. After the buyback, Apple is still worth the same $500 Bn, but with approximately 1/3 fewer total shares, Icahn's shares are worth about 33% more. He'd stand to make about $800 million by doing jack.
Icahn's idea has little to do with making Apple more valuable and everything to do with making himself richer.