I do IMap, have a desktop, laptop, and Ipad. Mail on Desktop and laptop all do nice things with junk mail... like get rid of them. The IPad does nothing that I can find so far.
I would suggest making Ipad Mail more like Mac Mail in terms of Junk. I get tons of mail on different accounts, but I generally have to wait to see them on the desktop or laptop. Yet they are still there because of IMap. I want the good ones to be there I just need my Mail filtering capabilty on the Ipad2