bug (feature?) in zsh

[Toshiro <toshiro@xxxxxxxxxxxxxxx>]

I'm using Debian unstable, zsh version 4.2.2. I've noticed that, when I 
install a new package that puts some program in my path (for 
example, /usr/bin) I can't invoke the program using completion in any zsh 
shell that I have opened before installing the program. 

Just an example to clarify this (my english is very poor :))

Let's say I install the program sdiff in /usr/bin; if I have another zshells 
open before installing the program (as is always the case in my box) and try 
to invoke the program pressing sd<TAB>, sdiff does not appear in the list of 
possible commands, I have to type 'rehash' to be able to see the new command 
in zsh.

I don't know if this is a bug or this is the intended behaviour, but I don't 
really like it. Anybody knows how to fix this? 

Regards,
Toshiro.