Some clarification about checking library holdings from online bookstores:
The check is being done by ISBN, so a bookmarklet can be created for any site (not just Amazon and B&N) that uses the ISBN as part of the URL for the record (e.g., one of my fave places BookCloseouts.com).
BUT, this means that if the library has the book in a different format (hardcover instead of paperback), and therefore a different ISBN, the record may not show. Of course, you can always click on "Other Editions" and check from those pages.
I still think this is way cool...