I use Joomla 1.0 for the IEEE Security in Storage Working Group (SISWG) homepage, and discovered that when new users register, their e-mail addresses are e-mailed to them in clear text before being hashed using MD5 and stored in the database. Since SISWG is a security group, it's important to provide a little better security than for the usual Joomla user. Things like sending a plaintext password in e-mail are a no-no.
Unfortunately, it looks like Joomla 1.0 does not provide a way to disable having the users' e-mail sent to them if the admin chooses to require registration. The closest thing is to change "Use New Account Activation:" to "No" in the Global Configuration->Site tab, but then users can register without a valid e-mail address.
Fortunately, this is just a one-line change to the appropriate file, which shouldn't be a problem for those who don't mind getting a little dirty. Here are the edit instructions:
Open the file components/com_registration/registration.php and change this line:
154: $pwd = $row->password;
154: $pwd = "********";
If you're not using version 1.0.12, the line number may be a little different.
That should do it!