One of the Inkscape Board's actions in 2013 was to establish a trademark policy for the Inkscape name and logo. With the Software Freedom Conservancy's help we've completed the registration and documented under the About menu item on the website:
The motivation for doing this was due to questions that kept cropping up from people wanting to use it for legitimate purposes (e.g. using our unmodified logo on merchandise), and from people noticing our trademark used illegitimately (e.g. selling unrelated software using our name or logo). We wanted a policy that would provide clear guidance for all these uses, that would be liberally permissive for non-commercial and FOSS-friendly commercial uses, while prohibiting obviously fraudulent uses.
Along with this, I've added a FAQ entry (below) with a brief summary.
== FAQ: How are the Inkscape name and logo protected? ==
The Inkscape name is trademarked under U.S. Trademark Law, and held and administered by the Software Freedom Conservancy. Our policy is available on the website, and describes the conditions under which you can use our name and logo. We've aimed to make our requirements reasonable and friendly: We want to avoid things like other groups claiming to be or represent us when they don't, or to distribute our software in ways that aren't consistent with our license.
The logo is similarly trademarked and covered by our trademark policy. We're cool with people creating derivative works that help promote Inkscape or make it fit in with a given user interface style, but we don't want to see it abused by companies that might incorporate it into their own logos. For merchandising, you are welcomed to use our unmodified logo; for anything else, please contact the Inkscape Board for permission.