What we're being cautious about is the UI stuff and wireframes, cause we're not sure how THAT is covered under copyright law. However, it's more than just begin over-cautious when it comes to code.
The aDPL has a clause regarding "giving credit to ArsDigita" which is extremely similar to the clause that the old BSD license had. This was *known* to be a sticking point with the Free Software Foundation and known to be incompatible with the GPL. Thus, it is fairly *certain* that the aDPL and the GPL are incompatible. The caution has to do with the scope of what's covered under this license, but where the discussion pertains to code, I'm fairly certain it's incompatible.