There is an excellent legal analysis of the liekliehood of success of the ACORN suit on the legal issues at Copyrights and Campaigns blog.
One of the commenters points out the case of Lachman v. Sperry-Sun Surveying Company, 450 F. 2d 850, in which a surveyor was checking an oil and gas well under a contract with the the leaseholder that prohibited the surveyor from disclosing the results of any work. The Surveyor discovered that the well was not vertical and the bottom of the well encroached the neighbor's property. The surveyor told the neighbor about the encroachment. The leaseholder sued the surveyor for breaching the nondisclosure obligation of the surveyor's contract. The court found that the public policy of the state would not permit a contract to be enforced that permitted a crime to go undetected.
I think the Baltimore Circuit Court should find a public policy exception implicit in the statute the forbids surreptitious recording, if the enforcement of the statue would have permitted a crime to go undetected. Of course, the court could also find that there was no expectation of privacy in an office with an open door where children could be plainly heard in the background.