@zwol @mjg59 I keep hearing this from time to time, but the only actual source I've ever found for that was traced back to _one_ person who worked at Sun (amongst tens of thousands of others) at the time claiming this.
On the other hand, the explanation given by others at Sun was that they did not have the rights to open source all of the pieces of Solaris, so licensing it as GPL would have meant no one but Sun would have been able to build Solaris. At the time, there was no interest from anyone to just open pieces of (i.e. just open source ZFS, etc). If anything would have led to accusations of 'oh, they're deliberately withholding pieces' for some perceived nefarious intent.
So Sun wanted a license that was copyleft, but at the file level to get around the problem of the bits they didn't have the rights to license so others could build Solaris (so it didn't give Sun any special advantage -- aside from having more experience with it). They also wanted something with a patent peace provision. NetApp had sued them over ZFS, and at the time IBM was somewhat notorious for bullying other companies with their patent portfolio, so these weren't exactly idle concerns. There wasn't an open source license at the time that fit the bill.
So on one hand, you can believe that everyone at Sun was all of one anti-GPL mind complete with pictures of RMS on dartboards in their offices, and did all of this just to spite the open source community, or you can believe that they made an honest effort to do the best they could given the constraints.
Having known some of the people that were actually intimately involved in the process, and knowing that they are in fact big advocates of open source, it always annoys me that they get shit on for trying to get the best results they could given the constraints they had because they weren't able to achieve 100% perfection.