The suit is over the trademark to Unix, a trademark owned by The Open Group, and whether Apple will have to: 1) pay a license to use the name; and 2) test their software for compliance with the Open Group's requirements.
The Open Group owns the trademark (which it acquired in 1994, according to their site), although I can't find any filings at the US PTO that confirm this. They are claiming that Apple has used the mark without rights.
Apple claims that the term, as used, is a generic term and therefore they don't need to pay the Open Group a cent.
According to the Open Group's site, there are currently 13 licensees that have been certified, including: AT&T, Caldera, Fujitsu, HP, IBM, NEC, Silicon Graphics, and Sun.
The trial is set to get underway by February.