Dan Messisco started an excellent thread on this on the Aikido Journal website here
. (Footnote: I've trained with Dan -- he's amazing.)
I've gone to aikido seminars sans hakama and wearing a white belt. Result? The people who gave me the hardest time (eg resistant ukemi, non-committed ukemi, not paying attention to what I was doing and looking bored) were the brown belted practitioners (1st/2nd kyu). I noticed I didn't have the same kind of issues with higher ranked folks; I remember being tossed into a koshinage by one 5th dan during jiyuwaza which was great.
(A friend of mine (from DC and now teaching up in Arlington, Virginia) recognized me and asked, "Jun, where's your hakama?" I told him I was just trying to go incognito...)