I've been to a few Borders in Massachusetts, and I've seen the books with my own eyes!
Also, my brother said he's spotted it in Tampa, Florida.
Here's a crummy cell-phone picture of a bookshelf in Boston:
In Borders, it's on Shelf F in the kids' section, "Friendship and Humor."
If you can't find the book in your local bookstore, please ask at the front desk, and they'll order it for you!
P.S. For a lot of schools, next week is Spring Break. What better way to pass the time than with a copy of Diary of a Wimpy Kid? :)