Gawd, I hate cover letters. It totally depends on how you're sending in the resume, IMHO. If it's an email submission, you have to have something because a blank email with an attachment is just weird. In that case I think short and straightforward is best. "I am a [job title] in the [city] area with [number] years experience in [thing 1], [thing 2] and [thing 3], and believe I would make an excellent candidate for your recent job posting."
If you're actually mailing it in, a longer one might be more appropriate. Write it as if you're answering the question, "What makes you an ideal candidate for this position?" and just hit the highlights. If your personality should be allowed anywhere, it's the cover letter, but never too much.
