It’s not where you’d expect it to be… Here’s where Unity 4 installer threw the demo project (Windows XP):
C:\Documents and Settings\All Users\Documents\Unity Projects\4-0_AngryBots\Assets\AngryBots.unity
I think the reason it ended in that profile is because Unity installed for “all user” on the computer, thus ended up in the default “All Users” profile’s documents.
I hope this helps others. If it does, vote it!
Enjoy!
Sirus
P.S.: This version of AngryBots is full of bugs…