The questionable commit:

    {
      // Add the first line of localized text...
      cupsFilePrintf(fp, "*%s.%s %s/", lang->language, ppd_option, ppd_choice);
      while (*text && *text != '\n')
      {
        // Escape ":" and "<"...
        if (*text == ':' || *text == '<')
          cupsFilePrintf(fp, "<%02X>", *text);
        else
          cupsFilePutChar(fp, *text);
        text ++;
      }
      cupsFilePuts(fp, ": \"\"\n");
    }

Can someone explain to me how this allows arbitrary code execution? As far as I can see, all it does iterate through a string and markup some special characters.

Edit: Okay, after reading the blog post, and this fantastic bug report, it sounds like to print to a CUPS server, you send it a message on port 631 using an IPP (some print protocol) server. CUPS then requests attributes of the IPP server, one of which being the print filter command to run ("Foomatic-rip") to use to convert a PS or PDF into native print code. By requesting attributes, an exploit involving string escaping through the use of unexpected spaces or quotes can override the Foomatic print command. Arbitrary text can be supplanted, which will then be executed by the CUPS server.