Mongoose has decided that, upon buying up all of the versions of Traveller, that they would actually be somewhat open with it.
Enter the Traveller’s Aid Society publications, Mongoose’s way of delineating which games you can use material from, and exactly what you can use.
The short version is that they will not allow you to create works of fiction (except vignettes to slip into rules) or software, but will let you create role playing materials based on the current (Mongoose second edition) and classic rules. You can use names, places, equipment, characters, and so on, but not artwork from Mongoose or other Traveller publications. Finally, you can charge for your supplement, it appears, but you are not required to.
No other editions of Traveller are eligible at this time.
Also, for Classic Traveller supplements, you are required to use the above template.
Generally speaking, since the rulesets themselves are not fully copyrightable, I’m not sure how much of a difference it really makes for people wanting to continue with Cepheus and other branches of the ruleset. I know several are not inclined to try their luck no matter that the Cepheus rules were released under an open license.
For those willing to use the Mongoose templates, this offers an opportunity - among other things nothing says you have to use the default imperial setting.