In November 2012, the Illinois Supreme Court adopted new Illinois Rule of Evidence 502 regarding subject matter 
waiver and issued its significant subject matter waiver decision in Center Partners, Ltd. v. Growth Head GP, LLC, 981 N.E.2d 345 (Ill. 2012). You may view new Rule 502 here and the Center Partners decision here.
In a recent article published in the Illinois Bar Journal, the authors explain the significance of the Supreme Court’s November 2012 actions on the subject matter waiver doctrine. See Judge Gino L. DiVito, Brian C. Hausmann, and John M. Fitzgerald, New Limits on Subject Matter Waiver of Attorney-Client Privilege, 101 Ill. B. J. 348 (July 2013). Noting that Center Partners and Rule 502 “strengthen the privilege,” the authors conclude that the November 2012 developments provide lawyers with greater predictability of the consequences of disclosing privileged information in extrajudicial settings–such as business negotiations–and to governmental agencies. The article, accessible here, is well worth the read.
For further reading on the subject matter waiver doctrine, see the POP post on the Center Partners decision and a POP post highlighting an IADC article on the decision. You may also find interesting this POP post referencing an article challenging the basis of the business-negotiations exception to the subject matter waiver doctrine adopted in Center Partners.