I'm running Debian testing on my box at work. 

apt-get update
apt-get dist-upgrade
apt-get install pixieplus
restart machine, to verify computer can be started in the event of a 
power failure.

kde has now been removed and is uninstallable.
(pixieplus is a kde package.) 

apt-get install kde
**lots of error messages, saying things like I need to install kdelibs3 
and koffice but it's not going to be installed.

kde was running ten minutes ago.  Thanks to apt-get pulling testing 
sources, kde is now not running.  How do I let apt-get revert to the 
older version of kde?  This much-needed feature seems to be an extremely 
popular one to keep completely buried in Debian, no man pages, no howtos 
explain this.  I keep hearing, keep your debs, it has the information 
you need to restore a botched install.  Great, how do I unbotch?

an acceptable answer would be, how do I install gnome?  at least I'll 
have some kind of gui at my desk until Debian testing gets its act 
together in two weeks.  apt-cache search gnome seems to pull up all the 
gnome applications, but there's not "just gnome" like there is with kde, 
and installing any gnome-only package doesn't resolve the dependency to 
include "just gnome".  I tried gdm, but it's not 'enough', as I can't 
get past the login prompt.

--Alexander