apt: "The following packages have been kept back:"

[JW <jw@mailsw.com>]

I sometimes have a hard time figuring out why apt keeps certain packages back. 
For example the other day during an apt-get upgrade it was "keeping back" 
openoffice (and all the OO componant packages). It was not requiring any 
packages to be removed, it wasn't causing any major packages to be upgraded. 

How can I find out the *reason* why a package is kept back?

This is all on a default Etch system - nothing peculiar about it, no unstable 
or backported or third party repos used, no special pinnings setup.

Thanks,

	JW