SCO believe that they have spotted code in the Linux kernel that comes directly from Unix source code. They believe that this code came from IBM, who have a valid license for Unix.
Initially, they are trying to sue IBM for copywrite infringment, but then they started delivering written warnings to big users of Linux, threatening court action, and probably have the big Linux distributers on their radar.
In retaliation, IBM have done nothing except say 'see you in court', and help Novell buy SUSE ... does it sound like they're on the run?
Things that complicate the issue:
BSD Unix - source code is public, could the code have come from there?
SCO themselves had their own linux distro, passing out this code themselves, so technically, the code is now GPL after being released by the 'owner'.
SCO have not shown the source code that is at issue, except some really obscure code that could go either way.
I think that SCO were hoping that IBM would settle. Now that is't going to court (early 2005 I think), they've gone into 'FUD' mode, spreading all the dis-information that they can in the hope that people who use Linux will buy Unix licences from them - the smart ones are waiting for the outcome of the courtcase