I took a 6 pack of Heiniken and a bottle of white wine in the last two weeks. But this activity was encouraged as we had leftovers from the open house.
I'd say a mouse might be crossing a line although I've "borrowed" some software from here before. If you find yourself rationalising taking home a monitor or the network server, then you might have some issues.
I used to jack all my graff supplies... sharpies, jumbo pilots, stickers, had a nice little scam going for a while returning supplies back to grand and toy / office depot for cash - now i'm working for a small company so I don't jack shit (haha)... i think if you're working for a large evil company it's you civic duty.