This Munchkin bookmark was initially available as an insert in Game Trade Magazine #145, but will eventually be handed out as a promotional item at conventions and tournaments and will also be included free with orders made through the publisher's online store, Warehouse 23.
The artwork on the front is unusual, in that the person on the right is Jeremy Carty as drawn by Ethan Nicolle (the Axe Cop co-creator). Jeremy's appearance is due to a charity auction, where the winning bidder of $752 got to choose who would be on a Munchkin bookmark. More details are available in the publisher's February 7th, 2012 Daily Illuminator.
The special in-game ability of the bookmark is the capability to force someone to help you in combat (but not for the winning level).
The rules are available at this link.