It seems that the Brits use this and consider it typically British:
- Alan Townsend, a British teacher of English, used this phrase in an ESL idiom test he made.
- Mister Micawber, an American teacher of English who has lived in Japan for a long time, appears to know it and finds it normal enough (same link), so it is probably not new in America either.
- It is mentioned as typically British in this list of British idioms.
However, they all seem to use it somewhat differently from the way you use it, only as "what are you talking about?" — there appears to be no connotation of complaining about a fuss. The Urban Dictionary agrees:
what are you on about?
it is a shortened version on "what are
you going on about?" or "what are you
usually used when someone is not
making sense for extended periods of
time or if you feel like you've missed
(Sic.) So it seems you were right to think that a participle was dropped; since "what are you going on about?" is also used in England, it must be "going". Perhaps it has branched out into new meanings in America.
[Edit] I have also heard other dialects and regions claim it, so we can't be sure of its origin.