Yesterday, billionaire hedge-fund manager Carl Icahn surprisingly announced in that he would return $1.76 billion in outside funds to his investors. Icahn had given no indication over the past few months that he had any intention of stepping down as a manager of other investors' money, so I find yesterday's move a bit suspect.

Icahn noted that his primary reason for stepping down as a fund manager was the probability of continuing gyrations in the market. In short, he chose to leave on a high note. But I'm still not convinced of his motives. You see, in a few months, the new Dodd-Frank Act will require hedge funds with more than $150 million in assets to register with the Securities and Exchange Commission, and document nearly every facet of their daily activities.

Though he'll no longer actively managing other people's money, the move will still leave Icahn in charge of his own $5 billion-plus portfolio, with which he can still wield quite a bit of power. That said, returning investors' money could have unknown consequences for the current holdings in Icahn's portfolio. Because he isn't required to report the amount of cash he has on hand, it's plausible to assume that he might need to trim or sell some of the positions in his portfolio to get the cash he needs to repay investors.

What's under the hood
As of Dec. 31, Icahn owned a number of large positions. Motorola Solutions (NYSE: MSI) made up the largest, at more than $1.9 billion. Biotechs Biogen Idec (Nasdaq: BIIB) and Genzyme (Nasdaq: GENZ) combined to represent more than $1.6 billion in assets. Chesapeake Energy (NYSE: CHK) and Lions Gate Entertainment round out the top five.

Of these five stocks, it would seem most logical for Icahn to lighten or sell his holdings altogether in Genzyme. Since the company has agreed to be purchased by sanofi-aventis in an all-cash offer, its future price appreciation appears limited.

I also think it's possible that Icahn may choose to lighten up on his Amylin Pharmaceuticals (Nasdaq: AMLN) position, which was his sixth-largest holding behind the five mentioned above. It would be very un-Icahn-like to shed a position that has nosedived 25% in a week; after all, he is a master at being the architect for change at troubled companies. However, Amylin may be damaged beyond repair, after reporting that its diabetes drug Bydureon, developed in partnership with Eli Lilly (NYSE: LLY) and Alkermes, failed to match the benefits of Novo Nordisk's (NYSE: NVO) Victoza in a new clinical trial.

In the end, I could be blowing smoke here. Carl Icahn could already have set aside the required cash needed to reimburse investors. Still, it pays to know what's in this billionaire's portfolio, and what it could mean to your own investments.

What's your take on Carl Icahn's exit from the hedge fund business? Is he going out on top, or running away from new legislation? Share your thoughts in the comments section below, and consider adding these stocks to My Watchlist so you can track them with ease.

Add Motorola Solutions, Biogen Idec, Genzyme, Chesapeake Energy, Amylin Pharmaceuticals, Eli Lilly and Novo Nordisk to My Watchlist.

Fool contributor Sean Williams does not own shares in any companies mentioned in this article. You can follow him on CAPS under the screen name TMFUltraLong. Motley Fool Alpha owns shares of Chesapeake Energy, which is a Motley Fool Inside Value selection. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy which is completely transparent.