I actually read a government publication over lunch a few days ago that explained what a prospective employer can and cannot ask an applicant. I think the link was here, in a thread about being asked for a photo? (forbidden BTW...)
As I have been an employer, my interest in this had enough basis to have me spend my lunch hour wading through page after page of beaurocratic gobbledygook...
This publication reinforced my previous decision to never ever ever again employ anyone.
But if I did, I would remain anonymous until time to fill out the W4 in triplicate.
