
On the odd chance some of you are wondering the same things, I've attempted to answer them here:
The first question has a fairly straightforward answer, although Federal regulation is involved so it's not "simple."
The big issue is that Icahn is an "activist investor" as defined by the Securities Exchange Act of 1934. Activist investors must submit a 13D filing if they are going to acquire more than 5% of a company's stock. More detail can be found here.
As per his letter to Yahoo's board, Icahn is seeking "antitrust clearance from the Federal Trade Commission to acquire up to approximately $2.5 billion worth of Yahoo stock"; at current prices, $2.5 billion would buy more than 5% of Yahoo's stock.
The second question is a bit more complicated to answer. According to The Wall Street Journal, Icahn currently owns "only" 9.9 million shares, not the 59 million widely reported. "How can Mr. Icahn hold the rest without actually owning them? It's because of an old strategy Mr. Icahn and other corporate raiders used in the 1980s, of matching put-and-call options," the Journal explains.
The Journal goes into the mechanics of the options trade, but Paul Kedrosky sums it up this way: "People like Icahn are notorious for playing call option tricks and parking stock with buddies to build large de facto positions without disclosing until last minute," i.e. until he's ready to declare his intentions.
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