I don't really believe she's died. Can you imagine the savage mockery she'd heap on Death, what with that ridiculous cowl, anachronistic scythe, and anorexic-chic figure? And she would have never sat still long enough to be rowed across the River Styx. I assume she got tired of kicking the Democratic Party's ass, so she abandoned the Ivins identity and has returned to her home planet, where the oceans whisper one-liners and the homes are built out of pithy aphorisms. Her humanoid form was never really that convincing anyway. Real people just aren't that funny. So once her mission was complete and the Republicans were cowed and a newer, tougher, braver breed of Democrat had emerged, it was time to drop the charade. She will be missed, though. All we have left are her columns, but in some ways, they are enough. Certainly more than we ever deserved.
Update: See also John Nichols' remembrance.