I've been tossing around the idea of 5-boxing Onyxia on my shamans. Yanamana and I have just about finished all the UBRS runs I've needed to complete the questline - just 1 more General Drekk kill to go.

I'll probably focus on dinging 70, then go attempt it. I'll let you all know how it goes, as I'm definitely going to be attempting it and recording the fight to post (assuming i don't suffer an embarassing loss =P).

On a side-note, Onyxia has now been solo'd by a dreamstate resto druid. No bear tanking or anything, just straight moonfire, insect swarm, and incredible healing and mp5.
http://www.warcraftmovies.com/movieview.php?id=76343
Note: Interestingly the guy playing this druid, Serennia, was also the top rated arena player in the world on his warrior last season, alongside his druid healer partner. Talented individual.

I don't have a link, but I heard a prot paladin solo'd it recently as well. He claims first to solo onyxia because the resto druid had a hunter wait by in case the trash mob respawned and pulled, which it did. I'd still count it as a solo.