Slashdot linked up this Gaurdian article noting there's something small but very strange acting on the Pioneer space probes:
The strange behaviour of the Pioneers - which swept by Jupiter and Saturn in the Eighties [sic] - was discovered by John Anderson and Slava Turyshev of the Jet Propulsion Laboratory in Pasadena and Michael Martin Neito of the Los Alamos National Laboratory.
They had been tracking the probes using the giant dishes of Nasa's Deep Space Network. By the time the two spaceships had swept beyond Pluto, they noted there were persistent anomalies in their trajectories. Every time they looked the Pioneers were in the wrong place. The effect was not large, but it was significant. Something more than the Sun's gravity appeared to have a grip on the craft.