Basic physics tells us that the radioactive decay of isotopes (like Carbon-14) is a constant, and this fundamental principle is the basis for several commonly-used methods of dating materials, such as Potassium-Argon dating of mineral samples, or the previously mentioned Carbon-14 for organics.
So what happens if radioactive decay is not a constant? That seems to be the implication of new
findings from Stanford and Purdue University, who observed a apparent seasonal variations in decay rates, which seems to be tied to the rotation of the inner core of the sun. They theorize that solar neutrinos - practically weightless, travelling at almost the speed of light, and able to pass through the whole planet - are the culprit.
How long before this is used by young-Earth creationists to "prove" that radioisotopic dating is wrong, and the planet really is only 6,000 years old? Any bets?