The basis for the lawsuit is essential the same as the lawsuit by Viacom against YouTube. Usenet.com is not a bad choice on the "inducement" prong of Grokster because they apparently promise access to MP3s (which, as we all judges know, is a code word for "pirated music").
I am not knowledgeable enough about Usenet architecture these days to know what happens if carrying Usenet becomes a liability. Google just developed a content match system for YouTube (although, as the recent flap over Moveon and Google ads highlights, there are still a few bugs and unintended consequences of the system). Maybe that can extend to Google Groups. But I do not know if the binaries groups are severable from the rest of Usenet, or if a court will even understand the distinction, or if a company will want to risk carrying it.
In a very fundamental way, I owe my career to Usenet, which educated me very early on about why a world of open communication with no intermediaries was way cool. I should be sad to see the RIAA kill it.