Those who have been wondering about the empirical status of "third wave" approaches will find a recent review by Steve Hayes et al (2004) useful. Hayes and his colleagues review the empirical support for Dialectical Behavior Therapy(DBT), Acceptance and Committment Therapy (ACT), and Functional Analytic Psychotherapy (FAP) and conclude that DBT and ACT each "have a small but growing body of outcome research supporting these procedures and the theoretical mechanisms though responsible for them" while FAP "has a limited research base, but its central claim is well substantiated." The authors don't explain why they chose just these three approaches to review. A number of other "third wave" approaches have decent empirical support as well. However, it looks as though this article gives a good overview of the available evidence.

Note: My only complaint about proponents of "third wave" approaches (not Hayes et al, 2004) it that they often talk as though empirical support for "third wave" approaches somehow invalidates more established CBT & BT approaches. Obviously, the finding that these approaches work does not invalidate all the evidence that established CBT and BT approaches work well with a broad range of problems. If proponents of these approaches stuck to saying that they've come up with interesting new approaches which we all may be able to learn from, I'd have no quarrel at all. For some reason, they often seem compelled to argue that evidence of the effectiveness of their approaches somehow proves that all other approaches are wrong, or at least hopelessly outmoded.

Hayes, S. C., Masuda, A, Bissett, R., Luoma, J. & Guerro, L. F. (2004). DBT, FAP, and ACT: How empirically oriented are the new behavior therapy technologies? Behavior Therapy, 35, 35-54.