I found the link to this article by Peter J. Fogel on Writers Weekly via the website of Angela Nissel, author of Mixed and writer for "Scrubs," so she knows. It's got some good, practical advice:

Your spec must be formatted correctly. It must look like it came from the show itself. No distractions here. Don't give the reader a reason to hate your script before they've even read it. A sitcom format is different from a screenplay, which is different from a one-hour drama. And each individual show has its own quirky formatting as well. Try to get as close to original as you can. To stand out, you must be better than the next guy. Sloppy, unprofessional formatting? You're done. D.O.A. End of discussion. Nexxxxxxt!

More here.