Julia Gillard has been ousted as leader of the Australian Labour Party by Kevin Rudd (the guy she forced out 3 years ago). It is vaguely possible that she will choose not to resign as PM and hang on as a caretaker until the election in September (she has promised not to contend the election if she lost the leadership challenge) but it is more likely she will write to the Governor General offering her resignation.

Update - yep she's being reported as having resigned.