http://fallout.wikia.com/wiki/User_blog:Ausir/Bethesda_sues_Interplay,_new_V13_concept_art
EDIT: New details have surfaced. Turns out there are actually two lawsuits - one over Fallout Online, and the other over Fallout Trilogy and other re-releases of the original Fallout games. See:
http://fallout.wikia.com/wiki/User_blog:Ausir/Bethesda_vs._Interplay_-_more_detail