Look at it this way:
1- As long as your AOS application is pending, the "Airport stamp" on your AP document is meaningless and can be safely ignored.
2- You can travel with the same AP during the validity period of the AP document itself (i.e. you must return before May 13 2009). The airport stamp is just a routine 1-year stamp but doesn't affect your lawful presence in the US as long as your AOS is properly filed and pending. The airport stamp also does not extend the validity of your AP document, you must apply for a new one if you intend to travel after May 13, 2009.