I’ve never noticed this before, but every major Mac OS X browser icon is round. ((Internet Explorer’s “e” icon is round, too, but I don’t have that anymore.))
I guess the reason is “the world,” but I still don’t really know what’s so “world” about browsers compared to email or instant messages. ((Also, Opera’s “O” looks really off-center.))

Don’t forget: Konqueror (round), Epiphany (round), and Galeon (not so round). There are also lynx and elinks (but neither have icons).
Isn’t Galeon basically another globe with a footprint on it? (like flock)
Thanks for the note about Epiphany and Konqueror. Sadly, I run Windows here at work. I just didn’t feel like waiting until I got home.