Try this phrase in Google: tinnitus phase cancellation
It doesn't look as if an immediate panacea is on the horizon, as currently the only successful trials have ocurred in people with "fixed frequency" tinnitus, but future implementation of the technique may well lead to positive results in people with other variants of the condition.
Taken from a link Google came up with:
During a preliminary experiment, our researcher subjected a tinnitus sufferer to a puretone which had previously been matched to the subject's Tinnitus frequency and intensity. The phase of the puretone was then adjusted over a -360 to +360 degree range. As the phase was adjusted, the subjects observed a beating phenomenon. This is the first time a mechanically induced auditory signal has ever been noted to interact with a phantom neurological signal (perceived Tinnitus) to produce a beat.
The first clinical research subjects to observe the beating phenomenon were licensed clinical audiologists that were also Tinnitus sufferers. The phenomenon was then observed in a number of regular clinical patients where the patient was subjected to a pitch shifting test. Subsequent work found that by fine-tuning the amplitude and frequency match, patients could reduce the perceived intensity of the beat. Hence patients are claiming partial cancellation of their perceived Tinnitus.