Interesting post recently by Universities Weblog about reference letters. Although most students have very similiar credentials, the post makes the point that what can often separate them out are reference letters, but many of those letters have taken on vague language that would make even the most skillful of Supreme Court nominees

Many professors and others on-campus will agree to write letters of reference for their students, but advice their students that their letters will have a greater impact if the writer sends the letter directly to the employer. The letter writer is therefore free to be more candid than they might be if the letter was first read by the candidate. Yet even though the letter is going to directly to the employer, many of the writers are increasingly choosing to use double speak when describing their students. For example, rather than describing Tommy as a poor student, the writer will state that Tommy works to the best of his abilities. Or rather than describing Sally as being cruel, the writer will state that Sally has few friends. So it isn't so much what they're writing, as what they're not writing.

Students and other job seekers: before you ask a professor or anyone else for a letter of reference, be sure to ask them whether it will be positive. Ask them to be candid with you and then listen carefully both to what they say and what they don't say. If they clearly indicate that they will provide you with a glowing reference, awesome. But if they tell you that they can say some nice things or that they wish that they knew you better, take the hint. For however vague that hint is, they're trying to tell you something by not telling you something.