... and as far as i can understand, any kind of use of the service has to be priory authorized directly by WTC, it's not only a question of logo and link to them, that means (i think) that each skinner have to get the WTC permission to do what he wants to do, widgets or so
Could you quote the section? I read the whole thing and don't see that anywhere. All I saw was this:
You may not modify, publish, transmit, display, participate in the transfer or sale, create derivative works, or in any way exploit, any of the content, in whole or in part. Except as otherwise expressly permitted under copyright law, no copying, redistribution, retransmission, publication or commercial exploitation of downloaded material will be permitted without the express written permission of TWC and the copyright owner. In the event of any permitted copying, redistribution or publication of copyrighted material, no changes in or deletion of author attribution, trademark legend or copyright notice shall be made and no ownership rights shall be transferred.
To my understanding, the bolded part pretty much authorizes a TWC weather widget as the TOS for the XOAP data feed expressly permits, in writing, the licensee to distribute Their Desktop Application.
In any case, if any author is uncomfortable with all of TWC's legalities (which is understandable after this debacle) they don't have to use a TWC widget. Myself and other DXers are looking into other weather sources for just that reason. No worries. 
