http://news.yahoo.com/s/ap/20100902/ap_on_bi_ge/us_illegal_immigrationof enforcement ( which I am in favor of), border security ( also of which I am in favor)
and the down economy ( of which I am not in favor of) I would bet the economy is the factor MOST responsible for this-- not solely, mind you, but mostly--
what needs to be done is work on making it economically unprofitable for companies to hire illegals while the ecnonmy (hopefully) imrproves--
in other words, reverse the trend that brought them here, then they will leave-- reverse migration.