http://www.gao.gov/new.items/d... is from 2005, but it should give you a pretty good idea.

well i dont know, if they have acces to IRS but wt i think is even if they have, they dont bother to do tht coz thts like hell lotta job to do, unless theres a serious criminal cases involved. i have most of my friends (F1 visa, student) who applied and received the work permit although they'd been working before using theirs socials and paying taxes. work permit for students is def. provided by DHS yall probably know tht. so, if they r suppose to run the IRS database things or wtever, they wud probably come to arrest those students applyin work permit who's worked n paid taxes before without DHS authorization.