I know there's generally not a ton of Oscar discussion on here, but what the hell. The Oscar's usually have at least one unexpected win or "upset" every year. The following nominees are considered "locks" and if someone else wins, it will be an upset.

Best Director - Kathryn Bigelow for The Hurt Locker
Actor in a Leading Role - Jeff Bridges for Crazy Heart
Actress in a Leading Role - Sandra Bullock for The Blind Side
Actor in a Supporting Role - Christoph Waltz for Inglourious Basterds
Actress in a Supporting Role - Mo'Nique for Precious: Based on the Novel 'Push' by Sapphire

I don't feel comfortable enough saying that there's a lock for Best Picture at this point.

Does anyone have any predictions about which category it will occur in this year? Or maybe that it won't happen at all. My guess is that it will happen in Lead Actress or Supporting Actor. I'd prefer it to happen in Lead Actress with Meryl Streep taking it for Julie & Julia. If it happened in Supporting Actor, I could see it going to Christopher Plummer, just because the Academy likes to award older actors every now and then.

I think you are right on with your pics for Best Actor and Best Actress. However in all honesty, I don't even know who the other nominees are. I would like to see Sandra Bullock win (even though I haven't seen "Blind Side" yet!) Frankly I haven't seen any of the films that are up for awards, which sadly is usually the case when it comes to the Oscar nominees. Actually most of those films I've never even heard of!
I've heard so much about Avatar that I wouldn't be surprised if it were to win. Not my type of movie at all, though.