It may come as a bit of a surprise that not everything about Blender is Open Source - the logo and the brand name 'Blender'. This is not uncommon for Open Source projects - the Mozilla Foundation and TYPO3 association (I'm an avid TYPO3 user) both do something similar with their brand and logo. The Blender Foundation have now published a set of proper guidelines for the use of the Blender logo.
Ton Roosendaal writes:
Blender's logo has been used in 100s of ways, in artwork, as smileys, as part of other logos in headers of community websites. This was - and still is - considered to be an honest tribute to Blender, and the guidelines are not meant to make all these versions "illegal" or "officially disapproved".
This page is only meant to clarify the official Blender Foundation guidelines, so people know what their minimum rights are how and when to use the logo.
Making fun and cool versions of the Blender logo is really part your own artistic freedom, and Blender Foundation will never act against such tributes. Just don't expect us to "officially approve" it, that's all. :)
Amsterdam, March 2009
Ton Roosendaal, Chairman Blender Foundation