The cogwheel is the complete logo, i.e. the logo which (hopefully by now) everyone uses to refer to RISC OS. The cube logo refers specifically to the early versions of RO4, i.e. RO4.0x and since then ROL have adopted the cogwheel as the basis for their OS logos. The RO Six logo is in my opinion a slightly better logo, where the aluminum / metallic 'SIX' seems to have been influenced by Apple's depiction of its Mac OS X Tiger logo. Incidentally, there seem several areas within Select 3 and later which seem to have been influenced by OS X style and behaviour. I guess it's the iMac G5 they have, though Apple seems to inspire a lot of OS design nowadays with Windows Vista being a particularly obvious example.
Castle has certainly also adopted the cogwheel, however it may not be quite as prevalent as is on ROL's website. ROOL has also very clearly adopted the cogwheel, although their version has been cleverly enhanced to illustrate the open and diverse nature of an open source version of RO5. So, I believe we have a logo and for some time now. For example, it's always used by OSNews.com to indicate RO related articles. However, perhaps in due time the cogwheel may be revised to gain a more attractive appearance.