I would say that while there are hypotheses with such low priors as to make it irrational to expend the effort to check them (and Harry would probably have assigned such a low probability to the existence of HP-style magic, and magical Britain, before he got the letter), one of these hypotheses being promoted specifically to your attention in the manner of the letter probably raises the prior to the point where it's worth testing, at least.
Talk to a prominent academic who gets lots of letters, and you'll change your mind. My advisor developed this index to use before bothering with the letters he's given. (The index itself is a joke, but the idea is real.)
Update: Discussion has moved on to a new thread.
After 61 chapters of Harry Potter and the Methods of Rationality and 5 discussion threads with over 500 comments each, HPMOR discussion has graduated from the main page and moved into the Less Wrong discussion section (which seems like a more appropriate location). You can post all of your insights, speculation, and, well, discussion about Eliezer Yudkowsky's Harry Potter fanfic here.
Previous threads are available under the harry_potter tag on the main page (or: one, two, three, four, five); this and future threads will be found under the discussion section tag (since there is a separate tag system for the discussion section). See also the author page for (almost) all things HPMOR, and AdeleneDawner's Author's Notes archive for one thing that the author page is missing.
As a reminder, it's useful to indicate at the start of your comment which chapter you are commenting on. Time passes but your comment stays the same.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: