With friends like Carl Icahn, who needs enemies? The activist investor has extended his IT playing field from Dell (DELL) to Apple (AAPL), buying a reported $1.5 billion stake in the iPhone and iPad maker and afterward taking to Twitter to prod Apple CEO Tim Cook that the company’s stock is “extremely undervalued.”

That, in a nutshell, is Icahn’s modus operandi—buy in, shake the corporate tree for change and, ultimately, walk away with a handsome profit. Specifically, Icahn is urging Apple to buy back shares now, thereby increasing the earnings per share owing to fewer shares outstanding.

Last year, Apple bumped a stock buyback program from $10 billion to $60 billion, a repurchase plan expected to run through 2015. Last month, the company said it already had ponied up some $16 billion to buy back shares in Q3.

But that’s unlikely to satisfy Icahn, who told The Wall Street Journal that he wants a buyback now at the current share price, contending Apple can borrow money cheaply to finance a repurchase.

"This is a no-brainer to go buy stock in a company that can go borrow," he’s quoted as saying. "Buy the company here and even without earnings growth, we think it ought to be worth $625," he said.

Apple stock rose 5 percent on the news of Icahn’s investment. With the company’s current stock market valuation standing at some $450 billion, Icahn’s stake, at less than 1 percent, won’t buy him near the influence he's wielded with his near 9 percent ownership in Dell.

Still, his reputation precedes him, which he used to his full advantage in announcing on Twitter his stock purchase and his “nice conversation” with Cook, making good on a press release in which he said he intends to use Twitter “to communicate with the public about our company and other issues.” It's a tactic Icahn's already adopted—on July 24, he used Twitter to poke fun at Dell founder Michael Dell, writing, “All would be swell at Dell if Michael and the board bid farewell.”

This time, in two tweets, Icahn wrote, “We currently have a large position in APPLE. We believe the company to be extremely undervalued. Spoke to Tim Cook today. More to come,” and “Had a nice conversation with Tim Cook today. Discussed my opinion that a larger buyback should be done now. We plan to speak again shortly”.

Apple, in acknowledging Icahn’s buy-in, said, in a statement to AllThingsD, “We appreciate the interest and investment of all our shareholders. Tim had a very positive conversation with Mr. Icahn today.”

Icahn’s interest in Apple reportedly rose the more he learned of the company’s huge $147 billion cash storehouse and its ability to borrow at low prices to back a large stock buyback. In addition, Icahn is betting on the vendor’s ability to continually produce game-changing technology, such as its long-rumored iWatch and Apple television, to drive sales, rather than grinding out updates to its existing technology.

At this point, there’s no telling Icah’s next move. But it would be a mistake to characterize his early friendly tone with Cook as setting the boundaries for their interaction. In his battles with Michael Dell, Icahn has not hesitated to use the press and open letters to shareholders to accuse him of any manner of ill behavior toward shareholders—a practice that has earned him stiff rebukes from Dell’s Special Committee overseeing the buyout.

Icahn recently bought 4 million more shares of Dell stock to up his ownership position to 8.9 percent of the company, second only behind Michael Dell’s near 16 percent stake.