No, AT&T already tried that, against BSD. They lost, big time. Interestingly enough, a major reason was because AT&T was found to have copied significant amount of BSD code. I don't know all the details, but you can find more info at:
http://cm.bell-labs.com/cm/cs/who/dmr/bsdi/930303.ruling.txt
SCO's claim about the invalidity of the GPL is hogwash. A first-year law student could demolish it. Copyright law grants the right to make a single copy in the absence of permission by the copyright holder. But, the copyright holder can grant whatever permission they want.
For instance, Microsoft grants OEM's the right to make many copies of Windows for installation on new computers. If the GPL violates copyright law for this reason, then many other commonly-accepted practices are also illegal.