A fairly good number of articles in the Academic Search Complete database mention the search and circumstantial evidence for Planet X, including an article in New Scientist (1/31/2009) by Govert Schilling who wrote the 2008 book The Hunt for Planet X (Springer [a respected academic imprint]). Physics Today (and others) reviewed the book in its Sept. 2009 issue.
That the hypothesized Planet X has not yet been found could soon change. In December 2008, the first prototype of the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) was brought into service at the Haleakala observatory on Maui, Hawaii. Soon, four telescopes equipped with the world's largest digital cameras, at 1.4 billion pixels apiece, will search the skies for anything that moves. Its main purpose is to look for potentially hazardous asteroids bound for Earth, but inhabitants of the outer solar system will not escape its eye.
Lots of circumstantial evidence, though, for its existence. It's thought that the Kuiper Cliff (the abrupt edge of the Kuiper Belt) could have been swept clean by the gravity of Planet X. Indeed, that brings up one of the criteria for planetary status (formulated since the downgrade of Pluto to dwarf planet): It must sweep its orbit clean of debris. That's why Planet X is thought by some to lie well beyond the Kuiper Belt, at least as far out as 60 a.u. (astronomical units); others think at 50 a.u.
As an interesting aside (which brings the matter literally back to Earth and much closer to my interests), I looked at an article about Planet X, in Monthly Notices of the Royal Astronomical Society (9/21/2002), which mentioned that the periodicity (at 30 myr) of Earth cratering and some fossil records has led to speculation that periodic changes in Planet X's aphelion and perihelion (closest and farthest points of orbit) might set off Kuiper Belt Objects, resulting in periodic comet showers. Watch out! It's the flybys that are dangerous.
Why speculate a government cover up? When the evidence is in, as it soon will be in 4-5 years, we will all know about any new discoveries through the academic pipeline. Could you imagine a massive conspiracy to keep anything secret that involves researchers in many countries who are NOT working for any government, who would love to have their name ballyhooed? That's one of the goals of the academic types: Gain status through research and publishing.