Far as I can tell using the images I made in Fireworks, the difference between a PNG and Gif are too minuscule to tell at the dimensions we are using however the file size is a 72KB difference.
Sure my connection can handle that difference no problem, but when we are talking about that difference multiplied by hundreds of thousands of user each day, that difference really adds up in wasted bandwidth.
72KB x 250,000 users = 18,000,000KB which is 14.3GB.
Of course this is a worst case scenario by having every user who comes here viewing that icon, but the littlest things do add up. ;)