Some US figures about prevalence of fraudulent documents:
http://www.cbp.gov/linkhandler/cgov/...doc_detect.doc Another link suggested 9/11 hijackers had modified documents but I appreciate some of the facts in that case are controversial to many (chances of two passports belonging to hijackers surviving a massive fire unscathed). One can be sure that if the authorities state the problem is X, the real figure will be much, much higher due to those who escape detection.