I should point out that the AFL-CIO is strongly opposed to the ‘guest worker’ portion of the bill. See http://www.aflcio.org/issues/civilrights/immigration/upload/ImmigQ&A200610.pdf. They also wish to transfer the burden of enforcement to employers rather than equally to law enforcement authorities in general, local or federal. The AFL-CIO opposes both the House enforcement only bill passed last year as well as the current bill, in its present form, in the Senate.
Which may be another reason why Reid let the Dorgan amendment be considered again, i.e., to placate the unions on the guest worker program.