Apache v. GPL

[W. Yip]

Hi. I have some trouble grasping why Apache license is incompatible with
the GPL.

Is it because of the naming restrictions in Apache constituting additional
restrictions that are prohibited by the GPL? Or is it because of:

========
Redistributions of any form whatsoever must retain the following
 *    acknowledgment:
 *    "This product includes software developed by the Apache Group
 *    for use in the Apache HTTP server project (http://www.apache.org/)."
========

The above sounds like the obnoxious advert clause in the dreaded 'old-BSD'.

Then again, how does an advertisement clause such as the above amount to
incompatibility with GPL?

My final question is this. When literature mentions 'compatibility', do
they refer to compatibility of licenses in a situation involving:

(i) dual licensing (eg. Perl under both Artistic and GPL); or
(ii) intermixing of code released under different licenses.

Help appreciated.