Francesca D'Onofrio Stamford: Francesca Donofrio Stamford on Apple! I can

Cue the Apple "iCahn" jokes. Apple's stock topped the $500 mark Wednesday for the first time since January. Shares were moving higher for a second day on the news that activist investor Carl Icahn has taken a "large" position in the company.

Shares crossed the psychologically important level and climbed as high as $501.39 in afternoon trading.

Still, the recent gains have boosted Apple's value by $30 billion in just two days ... to more than $455 billion.

Apple's (AAPL) stock has been on a roller coaster ride during the last year, as investors have grown worried about a lack of new products. Shares hit a 17-month low below $400 in April. Even though they've clawed back lately, Apple's shares are still almost 30% below the all-time high above $700 reached last September.

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Icahn, who is known for taking large stakes in companies so he can influence management to make changes, revealed his investment in Apple in a pair of tweets Tuesday afternoon. He said he believes the company is "extremely undervalued." He also said that he and Apple CEO Tim Cook had a "nice conversation," during which he pushed for a large share buyback soon.

According to news reports, Icahn's stake is valued at $1.5 billion and he wants Apple to increase the company's $100 billion dividend and buyback plan to $150 billion. The size of Icahn's position in Apple will likely be revealed this week in a regulatory filing with the Securities and Exchange Commission.

Apple said in a statement Tuesday that it appreciates "the interest and investment of all our shareholders, and confirmed that Cook and Icahn "had a very positive conversation."

Icahn isn't the first activist hedge fund manager to urge Apple to do more with its cash. David Einhorn slammed Apple for "hoarding" billions in cash back in February. Two months later, Apple announced its plan to return $100 billion to shareholders over the next three years through a dividend and share buybacks.