[CentOS] Add instantly active local user accounts *with* password using useradd -p option ?

Hi,

I need to setup a load of user accounts on a series of machines, for 
testing purposes. I'm using a script to do this, but the only problem I 
have so far: I have to activate them all manually by doing passwd user1, 
passwd user2, passwd user3, etcetera. The useradd man page mentions a -p 
option to define a password, but I can't seem to get this to work. 
Here's what I'd like to be able to do:

# useradd -c "Gaston Lagaffe" -p abc123 -m glagaffe

And put that line in a script, so the account is *instantly* activated. 
I tried it, but to no avail. Looks like there's some burning loop I have 
to jump through first :o)

No security considerations here for the moment, since it's for testing.

Any idea how this works?

Niki