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So far, bupkis. One of the orbital observatories (probably Hubble, it's been up longest) completed a survey which supposedly ruled it out. However, the history of "ruling out" isn't an auspicious one. ;') From the files:

Massive planet may lie beyond Pluto
Royal Astronomical Society News Release
October 7, 1999
Writing in the issue of the Monthly Notices of the Royal Astronomical Society published on 11th October, Dr John Murray sets out a case for an object orbiting the Sun 32,000 times farther away than Earth. It would, however, be extremely faint and slow moving, and so would have escaped detection by present and previous searches for distant planets... The object would have to be at least as massive as Jupiter to create a gravitational disturbance large enough to give rise to the observed effect, but currently favoured theories of how the solar system formed cannot easily explain the presence of a large planet so far from the Sun. If it were ten times more massive than Jupiter, it would be more akin to a brown dwarf (the coolest kind of stellar object) than a planet, brighter, and more likely to have been detected already. So Dr Murray speculates that such an object, if it exists, will be planetary in nature and will have been captured into its present orbit since the solar system formed, even though the probability of such an event seems low on the basis of current knowledge.