In a follow-up entry, Ebert agrees that it wasn't cool. He insists it wasn't unethical, but admits he shouldn't have done it. As penance, he watched the rest of the film and added a section to his review. "I will never, ever, again review a film I have not seen in its entirety," he says. "Never. Ever."
It seems to me that there has to be a middle ground here. There's nothing wrong with quitting on a film and then writing about it, perhaps shedding light on why you walked out. But you have to front it. And you can't claim it's a review -- that means no star ratings. I walked out of The Mummy: Tomb of the Dragon Emperor and wrote about how awful it was. I don't think I did anything wrong. But I didn't pretend to be reviewing it.
That said, Ebert's addendum to his original "review" is characteristically insightful and well worth reading. And by the way: has anyone actually seen Tru Loved? What's its deal?