Is it experimentally ruled out that right-handed neutrinos are actually antineutrinos, and left-handed antineutrinos are neutrinos ?
Short answer: Unknown.
Slightly longer answer: the situation you describer would obtain if neutrinos were Majorana particles (and thus not Dirac particles). It is favored by theorists because it feeds into a nice explanation of why the neutrinos are so light by comparison to the other massive particles.
Experiments are underway that might settle the question by detecting so called "neutrino-less double beta decay" reactions which are forbidden for Dirac neutrinos but allowed for Majorana neutrinos.
June 2012: One of the neutrino-less double beta decay experiments, EXO, has announced modestly significant limit on the possible masses of the neutrinos if they are Majorana particles. This limit excludes some of the theoretically favored scenarios. Right now the significance is too low for a lot of fanfare, but this might be a the first indication of a surprise.
January 2013: KamLAND-Zen has reported a slight improvement on the EXO result as of the end of their first run, with purification of the working material planned before resuming data taking.