I've heard a lot about this in Pixar films, but I hold onto the belief that they're all just a bunch of Easter eggs. Fun to search for and point out, yes (especially when you consider the fact that some Easter eggs reference films that didn't exist yet at the time of their release - like the hidden Nemo in Monster's Inc), but trying to pass this all off as one, big universe leaves way too many plotholes. 

They're just a sign of the creators having fun with their work and giving the fans something fun to search for, as well. Even I enjoy that sort of thing - as on multiple occasions I'll be writing a roleplay post and see an opportunity to reference an older roleplay, and so I go for it. I'll do it even if not a single person in the current roleplay was in the old one or knows it exists, because I personally find it amusing, and it would be even [i]more[/i] amusing if someone actually did spot the reference.

Is this me attempting to string together all my RP's into one, extended canon? No, it's just me having fun - and justifying all my RP's existing in the same universe would be near-impossible even if it was my intention.