For better or worse, Yahoo! (Nasdaq: YHOO) has a vocal investor on its rolls.

David Einhorn's Greenlight Capital has amassed a stake in the meandering dot-com giant. Now let's see if he's going to roll up his sleeves to get the company on the path of improving its shareholder value.

"Our ideas start with a story," Einhorn explains in a interview earlier this year. "Is the market missing something that we see? Once we find an idea, the due diligence starts. We try to uncover a mispricing, and then we invest. Despite our reputation, we are predominantly long investors."

Einhorn's reputation? The guy's a hedge fund legend. If his name is ever dragged through the mud, it's usually when he's making a bearish wager. He can be relentless when he's in attack mode. His battle against Allied Capital (NYSE: AFC) was compelling enough to warrant a book. His more recent back and forth with value maven Bruce Berkowitz on St. Joe Co. (NYSE: JOE) was classic.

Yahoo! shareholders will be stoked to see Einhorn on their side. Yahoo! management may not feel as receptive.

Greenlight points out that Yahoo!'s investments in Yahoo! Japan and China's Alibaba are worth roughly $8 a share. I don't think the market has ever missed that. Yahoo!'s reluctance to unlock that value through either a spinoff or sale has made it difficult to appreciate. There's also the fear of what Yahoo! would be worth if it lost its sexy passive stakes overseas.

Yahoo! is one of the few Internet companies -- if not the only dot-com outside of AOL (NYSE: AOL) -- projected to post a dip in revenue this year. Yahoo! has sold off some assets, but the meatiest contributor to its top-line slide is its decision to let Microsoft's (Nasdaq: MSFT) Bing take over its paid search business. It's a move that will generate a lot of hands-free royalties over the next few years, but at the expense of Yahoo!'s once-ballyhooed brand in search. Revenue before traffic acquisition costs fell 6% in Yahoo!'s latest quarter -- a sharp contrast to rival Google (Nasdaq: GOOG), which saw its revenue soar 27% during the same three months.

Yahoo!'s display advertising business did climb 10% in its latest quarter, but will it be sexy enough to support the stock if its Asian investments are carved out? Will a management shakeup be required? Is Yahoo! about to become the next teetering tech darling to initiate a dividend distribution policy to smoke out income investors?

Things are about to get interesting.

What would you do to turn Yahoo! around? Share your thoughts in the comment box below.