True20 is Open Game Content, which means that you can use parts of it in your own d20 products if you obey the requirements of the Open Game License. However, the OGL is more restrictive than standard trademark law in that you can't indicate compatibility with a trademarked product unless you obtain a license from that trademark's holder. This is the difference between the OGL (free use) and the d20 License (which requires fees for indicating compatibility with d20 but allows you to use the d20 logo).
True20's title and logo are trademarked, so you would need to obtain a True20 license from Green Ronin in order to indicate compatibility with it or use the logo. GR will be giving a royalty-free license to each winner of the True20 setting search
for further support for their settings, but all other
publishers would need to obtain the license and pay royalties. (And, as written in the search rules, even the winners would need to pay royalties for any True20 material not tied to the selected settings.)
For reference, WotC's website has the text of the OGL
, and a FAQ
explaining certain parts of it.