According to this announcement on the official Debian website, Debian logos may be used freely for both non-commercial and commercial purposes.
In addition to this, the final artwork for the upcoming release 7.0 codenamed ‘Wheezy’ has been published too, just a few days after the Debian Installer RC1 was released. The artwork is following a light-blue pattern. You can download an archive containing the artwork from here. The package contains PNG and SVG files for the background, grub, the Debian installer, syslinux and even artwork designed for merchandising purposes (like T-Shirts, mugs etc).
