The new Google logo, on the google homepage, is 13584 bytes. And actually, using just ZopfliPNG, you can't get much further than that. #zopfli
However, with pngout (the industry standard before Zopfli was invented), it gets down to 12716 bytes, which is 6.4% better. #pngout
I noticed that the 24-bit image only has 682 unique colors, most of which are used for antialiasing the edges, so I tried a lossy reduction to 256-color palette + alpha. I can't tell the difference myself, so any visual differences seem quite insignificant.
After the lossy reduction, with the help of truepng (a scary Russian PNG optimizer) and zopfli, we're down to 7120 bytes; a 48% reduction in size.
(See http://en.css-ig.net/ and http://css-ig.net/scriptpng for more info.)
On the Google front page, a 48% size reduction of their logo should make for quite significant bandwidth savings.