Filed under: Private equityThis week, the $5.3 billion buyout of Ceridian (NYSE: CEN) has gotten a lot more complex. The company's biggest shareholder, Pershing Square Capital Management, is not happy about the price tag. More and more, private equity deals are coming under attack by activist shareholders. Then again, it tends to be an effective strategy.As for Pershing, it has hired the investment bank, Lazard (NYSE: LAZ), to help things out.Pershing's William Ackman has written a letter to Ceridian shareholders. Basically, he is going to seek out other suitors for the company (including strategic buyers and private equity firms). He would even be amenable to a big-time dividend or a spin-off of non-core divisions.All in all, Ackman is just trying to get a higher price.But it didn't have ...