Thank you for taking the time to find some links for me, monkeyshine. Having read them, neither one substantiates anyone's claim that rabbis are retroactively nullifying conversions. The first source merely discusses whether it is possible halachically (which isn't supported) and the second source, is from a "Progressive Jewish" action committee in Israel, and again, doesn't give any real reports, but merely claims, for obviously biased reasons, that Orthodox rabbis are giving people a hard time for no reason and yanking conversions willy nilly.
It sounds a lot like this whole discussion is based on some angry rumors by non-observant Jews who just don't like Halacha.