For what it's worth, I thought maybe I was on crack or something for thinking most readers I know don't mind notes at the beginning of fan fiction chapters -- it wouldn't be the first time I've been mistaken in my impressions, lol. So I pitched the question to the readers on my private blog earlier (which includes folks from the original fiction writers group I belong to), and so far all have said they're fine with notes at the beginning unless they're long. Then again, most of us are ex-Potter or Doctor Who people, so maybe we're a different stripe. Like I've said, readers and writers of the Naruto fandom seem to have very different tastes and expectations than what I'm used to, and I'm still trying to adjust.
Out of curiosity, have you written for other fandoms, tricksie?
You got me. No other fandoms. Only professionally.

Without going into detail, it has been my job for a lot longer than you've been writing fanfic to know what the reader wants, determine the visual heirarchy that will snag a user and keep h/her there, and organize the rest of the content accordingly. For newspapers, books and websites. I've happily shifted gears in my life, but the work ethic clearly remains, even in my personal projects like fanfic! lol!
Like I said, it may sound like I'm being overly concerned...but when you know you only have one chance to catch that reader, why bog it down with pithy or flat out silly author's notes that skew your reader's judgement of your work before the first word? In noveling, so much emphasis is put on the first sentence, first sentence, first sentence. But with fanfic, in a serialized form, the "first sentence" happens again and again. You are just as likely to gain new readers as you are to loose them with each update.
Each update, new readers make snap decision about whether to read a story that has a certain amount of reviews/words to their liking. They make decisions to read based on an author's commitment level to the story.
And each update, old readers make the decision to stick with the story based on how caught they are by that new "first sentence." Be it chapter 5 or 25. Is it still worth reading? Are they turned off by the direction it's going? How many times have you read an author's note at the beginning of a chapter to see things like "I've got a poll up," or "I've decided to do a time skip," or that dreaded (or beloved) word, "harem." Boom. Right there. You're done. No more reading. It doesn't matter if it's the best chapter you'll read out of any fanfic ever. You've already made your snap decision without dipping into the content at all.
Young writers don't realize what a disservice they do themselves by slapping author's notes at the beginning of the story. Depending on the whims of the readers, the story itself may never get a chance.
I'm not trying to be argumentative. It is only one of many factors that go into a finished work, and is a fairly unique problem to ff.net. But if you've worked so hard at writing something original, something that people want to read, why chance it by putting in something at the beginning that might turn them off?