Via View From The Wing and this FlyerTalk thread, Air Canada's Aeroplan appears to be pricing business class awards at the economy level, and first class awards at the business level. It started last Friday December 2, and most people thought it was just an award system glitch, but now there is word that this is in fact a "special promotion" that has been confirmed by people affiliated with Aeroplan. Not sure how accurate this confirmation is, but as of today, it looks like the discount is still on.
I had thought about visiting Toronto next summer, and I am currently still seeing SFO-YYZ business seats going for 25,000 Aeroplan miles roundtrip! Be aware that Aeroplan does tack on fuel surcharges to Air Canada award tickets as well as some partner airlines, which can add up quickly. But if you have been planning an award itinerary for sometime next year, I would take advantage of this massive discount right away. There is no word yet on when this "promotion" will be pulled, but I don't expect it to last more than a few more days, if that.
Remember that American Express Membership Rewards is a transfer partner, and miles are deposited into your Aeroplan account almost immediately. This is a great way to burn some of your MR points, especially since Continental is no longer a partner and British Airways has massively devalued their award chart.
EDIT: Seems like they are gradually fixing the issue. There may still be some business and first class seats at the lower level, but most are back to their normal pricing. Take advantage of this "sale" while you still can!