A while ago I suggested a logo for ALT.Net, which I really liked, but had copyright issues. Since then, I had commissioned the creation of a new logo, using the same ideas, which can be used without copy right issues.
This is not the official logo, there isn't any to my knowledge, but it express the way I think about ALT.Net very well.
The logos are:
You may use and modify the logos under the Creative Commons Non Commercial license.
I put all the raw images in this location, which can be used to adapt the images.