Yep, Pixar does that ALL the time. It's some sort of easter eggs, inside jokes.
But boy, some of those references are REALLY hard to notice.
For example, you can see Wall-E on Cars. Also, Wall-E seems to be on Toy Story 2 too.
Dug, from Up, also appears in Ratatouille and so on. And don't forget Lotso (from Toy Story 3) on Up too.
There are MANY MANY things like that. It's crazy, really. But I love it.