Yes, pretty much all airlines are overselling. However, Air Canada employees speaking on condition of anonymity are stating that business as usual for the airline is to string customers along until the last minute, even if it is known well in advance that they are not going to be boarding. That is to say, passengers with GTE on the gate indication of the boarding pass will not be boarding but are lied to and told that they will be assigned a seat at the gate.
Regardless your views on overselling, you probably take a very different view about being lied to about your likelihood of boarding a flight as you check in.
Frequent travelers know that getting a boarding pass with a seat number actually a race they don't want to lose, and will hound the call center if there's a problem with online check-in.
† Personally I think that airlines regulated and given two options for overselling flights:
1) If you miss your flight, and a paid passenger takes your place, you should be informed and not suffer financial consequences (re-booking fees, etc).
2) If the airlines has already sold a full flight and starts knowingly overselling, it should be communicated to the passenger when they buy the ticket ('priority standby' or something). This would, of course, make customers value these tickets less.
I think the EU regulations regarding denied boarding are pretty good (or at least a good start).