dapper and edgy mixed

[Marco]

Hello,

I to install a few selected edgy packages where the versions of dapper are
not sufficient.

Therefore I have the URIs of dapper and edgy in my sources.list. My apt.conf
is:
Acquire::http::Proxy "false";
APT::Default-Release "dapper";

This is a setup that works fine under Debian. But under Ubuntu there are
Problems:

1. adept seem to ignore the APT::Default-Release in apt.conf. It suggest to
upgrade all packages to edgy.

2. apt-show-versions -u shows all packages as upgradeable. I think this is
because it does not recognise the distribution of the installed package.
("/unknown"). Just one example:
mplayer-686/unknown upgradeable from 2:0.99+1.0pre7try2+cvs20060117-0ubuntu8
to 2:0.99+1.0pre8-0ubuntu4

What is wrong?

How can I maintain a mixed system?

Regards,
Marco