Decay Rates are not Constant?
Ok, now that I have the attention of the young earth creationists, I can report on a just published study in the journal Earth and Planetary Science Letters (Nebel et al., 2011 "Evaluation of the 87Rb decay constant by age comparison against the U–Pb system, v301, p.1-8).
The paper is significant because it proposes the first revision since 1977 for the decay constant of 87Rb. Steiger and Jager (1977) calculated a decay constant of 1.42 x 10-11 yr-1. Nebel and colleagues used analyses on the same rock samples to recalibrate the 87Rb decay constant by comparison with the U-Pb system. The 'new' decay constant is 1.393 x 10-11 yr-1. That may not seem like a significant revision, but it means that age determinations using the 'old' constant are off by about 2%. Sorry creationists, but the new constant makes things about 2% OLDER so definitely not a find in your favor. Perhaps more interestingly, it appears to be moving back toward the 'old' standard of 1.39 x 10-11 yr-1. We'll have to wait and see if the new 'constant' holds up to further analysis, but certainly in the examples given by the authors it brings the U-Pb and Rb-Sr ages into better agreement.
Oh and just in case you take refinement of this numerical value to signify that decay rates are not constant, that is not what the paper is about. It's about refining the exact value of the decay constant which is subject to both analytical and experimental error. Nevertheless, this isn't going to result in the Earth becoming 6000 years old.