I saw Toy Story 3 about two weeks ago, and I've been thinking: are Pixar movies, with the exception of Cars, all in the same universe? You can see Pizza Planet trucks in other movies like Finding Nemo, and I've been wondering if Bonnie from Toy Story 3 is actually Boo from Monsters Inc. What are your thoughts on this?

I think it's more likely that they've used familiar elements from one film in another more as a fan service than anything else. If correctly used, when people spot things like that it makes the film a little more familiar and likely to stick in the mind (as it has with you).

Yup, that's it. They've always done this. There are several things that keep cropping up, most notably the ball with the star on it from their very first short, Luxo Jr. That one is in every Pixar movie as far as I know. Also, John Ratzenberger (also known as Cliff the mailman from Cheers) always has a part.

The hinge lamp always finds a way in as well.

I'm pretty sure the Pizza Planet truck is in every Pixar movie. Strike that. I AM sure it is lol! I remember seeing a youtube video showing it! :)

Yeah that's why I wonder if the films are in the same universe. Do you believe Bonnie is Boo?
It's an interesting theory, in my opinion.

Not sure if they are or not, but it is an excellent marketing scheme for those that follow such things to see each movie to look for connections; smart.

Well, yes, since they incorporated it into their logo, that's pretty much a given.
There are other crossovers as well. The short film Geri's Game features the old man who restores Woody to prime condition in Toy Story 2.
Also, one of the "outtakes" of Toy Story 2 features Flik and Heimlich from A Bug's Life.

you could be right about the bonnie and boo thing though, pretty good observation. apparently boo has some toy story dolls in her room in monster's inc.

She does? I'll have to watch it again.

Boo also has a Nemo plush in her room...and the T-Rex from Toy Story appeared in one of Monster's Inc. 'bloopers' during the credit.

The aforementioned ball is also in her room.

Boo is in Monsters Inc? Does Minsc know?
*Slinks out before anyone can smack him*

Hah! Minsc is probably disguised as Sullivan. And who knows an adorable little girl is actually an adorable giant space hamster.
Monsters Inc is definitely my favourite Pixar movie. Ratatouille and Finding Nemo second fav probably.

Up is my favorite, because of the opening 10 minutes.