In my experience, depends on the individual agent as to whether or not they will make it a big deal. I travel internationally quite frequently (150000 miles on already purchased tickets this year) from the USA to Europe and Far East. Although my passport had to be modified with the addition of supplemental pages, never a problem until once, about 6 months ago upon return to ORD, an agent stated that the laminate in the corner close to the photo was "breached". After a consult with a supervisor, inspection with magnifying glasses, etc. I was allowed to re-enter but admonished that I must immediatly acquire a new passport.

Well, after many additional trips in and out of US, no problems...yet. (I am just reluctant to pay for a new passport and go to the time/trouble/effort to get a new one when I still have two years to expiration. However, I recognize the risk and carry a second, cancelled US passport as sort of a back-up in the event some agent demands I prove who I am, where I have been, etc. ( This passport was cancelled in June, 1998 when I received my now current one.)