As official as we'll see for now:
Bulls have also now informed Ronnie Brewer that his $4.37 million option won't be picked up for 2012-13.— K.C Johnson (@KCJHoop) July 10, 2012
The 'also' is in reference to what was reported similarly regarding CJ Watson's option (it's actually a non-guaranteed contract, which is slightly different but not that important of a distinction) after the Hinrich signing. The Bulls have a deadline (self-imposed, perhaps) of the decision on Kyle Korver this Sunday.
After the Watson report, the possibility of a trade was still mentioned. However, any trade possibility means having to first guarantee the contract this coming season season. It's possible, but this 'informing' of the player seems to indicate they're just going to become unrestricted free agents.