I don't think they can use ACORN to actually work on the census count. Those jobs are filled from applicants who have taken an exam, and further exams for supervisors. Works about like civil service and gives veterans preferences, etc.
The article says work with community organizations, which probably means the census workers would consult with ACORN to help locate everyone, legal and illegal.
And, there are no questions about citizenship on the census forms. Some illegals just avoid the census out of fear it might be used to find them, but it isn't used that way.
I applied and passed the exam and they said they would contact me in Feb. or March. I have not heard from them.