Bill Ackman's Pershing Square disclosed an 11.3% stake in Ceridian Corporation (NYSE:CEN) today in a 13G filing with the SEC. The stock moved up more than 5% in early trading this morning on the news. Pershing Square is a well known hedge fund that tends to take an activist stance in the companies in which it invests - most recently, McDonalds and Wendy's. Currently, Perishing Square has only filed a 13G indicating that this is only a passive investment. However, activist hedge funds occasionally use this filing type to acquire shares quietly before taking a more active stance, since it does not require a "Purpose of Transaction". If and when Pershing Square takes a more active stance in the company, it will be forced to file a 13D with the SEC which will outline its investment objectives. Given Bill Ackman's background, this is a distinct possibility, and definitely a stock worth watching.

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