actually, I think it's four circles joined together -- in Basecamp, circles are a repeating pattern used for people, so four circles are meant to symbolise a group of people -- indeed, a meaningful icon, just as you point out. Perhaps more meaningful than you were initially aware of! :-)
I hadn't noticed these elements before (except the circle). 37 Signals did a good job of introducing a consistent visual language. I love their concept of sheets.