On October 22nd, Mark Pilgrim and I announced the availability of a RSS validator built from the ground up to support all versions of RSS.
Both in the development effort itself, and in examining both feeds that failed, and feeds that one aggregator developer or another wished would fail, we became frustrated by a number of ambiguities in these specs.
Contrary to what some would have you believe, pretty much all of these ambiguities apply to all RSS versions.