I finally just became a CAcert assurer, and I thought I'd start spreading the word a bit. Just one problem: not really a large choice of icons/banners/buttons. After browsing a bit around what I could find, I just "designed" my own. Here's the "high-res" version of it (can't really do larger since I used the largest logo I was able to find....), it can be nicely scaled down to a more buttonly 106x32.
Edit (21 Jul 2012): Arf, just found a much higher resolution of the logo... Bah, that's big enough for a button anyway. Still here it is, for the next time: http://www.benny-vs-web.de/wp-content/uploads/2010/09/CAcert-logo-colour-820.png