Roudi said:
To the best of my recollection, the OGL 1.0a does not prohibit listing the precise source of a particular piece of material in the body of the text - the license only governs the citation of that source in the Section 15. trancejeremy's point about possible conflict with the "claims compatability" part is valid, but it's a matter of both interpretation and how that in-body citation is written.
I suppose that would be how Paizo "gets away with it". It isn't restricted to begin with, and they don't write it in a way that others interpret as a claim of compatability.
Most products list the name of the product as product identity, which means its protected under the license and can't be used.
For example, in Modern20, the single only thing we declared as product identity was the name Modern20.
Edit: Also, in Section 7 of the license (Use of Product Identity), it specifically prohibits naming another product without express consent of the company that wrote that product.
You agree not to Use any Product Identity, including as an indication as to compatibility, except as expressly licensed in another, independent Agreement with the owner of each element of that Product Identity. You agree not to indicate compatibility or co-adaptability with any Trademark or Registered Trademark in conjunction with a work containing Open Game Content except as expressly licensed in another, independent Agreement with the owner of such Trademark or Registered Trademark. The use of any Product Identity in Open Game Content does not constitute a challenge to the ownership of that Product Identity. The owner of any Product Identity used in Open Game Content shall retain all rights, title and interest in and to that Product Identity.
Edit 2: And because I'm sure someone will mention that few products actually contain real, actual, registered trademarks, the definition of that term within the license is somewhat broad:
"Trademark" means the logos, names, mark, sign, motto, designs that are used by a Contributor to identify itself or its products or the associated products contributed to the Open Game License by the Contributor
In other words, for purposes of the license, company and product names are automatically designated Product Identity and trademarks.
Edit 3: And I think doing away with these protections would involve some serious reservations on peoples' part when using the license.
Monte Cook's name (for example), or Mike Mearls', have a lot of value, and being able to sprinkle their names, and/or the names of their signature products throughout a book or on the cover would be seriously misleading.