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).
According to the current Debian Project Leader, Stefano Zacchiroli, “Software freedoms and trademarks are a difficult match. We all want to see well-known project names used to promote free software, but we cannot risk they will be abused to trick users into downloading proprietary spyware. With the help of SPI and SFLC, we have struck a good balance in our new trademark policy. Among other positive things, it allows all sorts of commercial use; we only recommend clearly informing customers about how much of the sale price will be donated to Debian.”
Here are a few screenshots: