Ringslinger, this charge of yours is blatantly false: "Not so, if they indicate that they have any inclination of voting for McCain, the registration form probably goes in the round file. On top of which, most of their registrations aren't legitimate."
In 2004 a Republican-backed voter registration was convicted of systematically destroying the registration applications of people who signed up as Democrats. This is one reason why ACORN makes a point of turning in ALL the applications they receive -- even the ones from Mickey Mouse or Joe's Plumbing; many states require that and where they don't, the organization would be subject to charges such as yours if they didn't.
ACORN has not, to my knowledge, even been charged with having destroyed or discarded voter registration applications.
As to "most of their registrations" not being legitimate, that's false. Out of well over a million applications (about 1.7 million, I believe) they've turned in, somewhere between 5 and 10 percent have been found invalid -- and most of those for things like clerical errors or missing information.