Jack Shafer criticizes him for threatening libel suits. I think he is largely correct. Unless someone publishes an outright claim that Moore is a liar and specifically identifies the lie he allegedly made, lawsuits are inappropriate. A better way to have this battle is in the arena of public opinion.
What i want to see is a documentary deathmatch between Moore and, say, O'Reilly: each of them gets to put together whatever they want for half the time of a combined documentary, like a debate with the ultimate in audiovisual aids. They get to put together whatever evidence they want to prove the other one has been lying or defend themselves from attacks. The time could be split up into alternating segments like 5 minutes of Moore, 5 minutes of O'Reilly, 25 and 25, 25 and 25, and 5 and 5 to close off, for example. A third-party organization could accept the segments and send them back and forth so each side could reply to the preceding segment if it wished.
The result would be a lot of fun, amazing to watch, and i bet it would have an even bigger opening weekend than Fahrenheit 9/11, because fans of both Moore and O'Reilly would show up hoping to see the other one proved a fool. Everybody wins.