It may very well be undervalued, but to say it's half-price I just cannot agree with. If Apple's price were to double, that would put it's market cap at roughly $1.2 trillion, which would make it the 14th biggest economy in the world based on GDP, right after Spain and right above Mexico. That would also increase it's P/E to over 32, which is well above average for mature companies like Apple, and in line with many small cap growth stocks. A P/E that high should indicate Apple's growth rate is consistently going to increase at a healthy percentage, which is really not possible for them because of how big the company already is.
Having said all that, I'm quite positive Carl Icahn has done his homework, and done it better than I have...but he also has different motives.
(all in my humble opinion)