"The Netscape Public License, or NPL, as it was ultimately designed in 1998, is a free software license--but it has three major flaws. . . . Two of the flaws apply to the Mozilla Public License as well. Because of these flaws, we urge that you not use the NPL or the MPL for your free software."
. . .
"3. Not compatible with the GPL...
This conflict occurs because the GPL is serious about copyleft: it was designed to ensure that all changes and extensions to a free program must be free. So it does not leave a loophole for making changes proprietary by putting them into a separate file. To close this loophole, the GPL does not allow linking the copylefted program with code that has other restrictions or conditions--such as the NPL."
from http://www.gnu.org/philosophy/netscape-npl.html
Essentially the GPL says that the restrictions it places on code are the only restrictions which may be placed on the code. Intentionally incompatible, it's a philosophy thing.
As for having OpenACS 4 use ACS 4.6/5.0, I'm surprised to see this is a concern since I thought everyone here felt aD was headed in the wrong direction. I don't know if any options exist for using ACS4.6/5.0 on a GPL project beyond writing modules that run on ACS 4.6/5.0, which you can then distribute with whatever license you'd like. (See flaw #2 at the previous link, which I don't see as a flaw).
