At least on my system (Windows XP), the keyboard shortcuts for this feature from Firefox 2 no longer work in Firefox 3.
For example, in Firefox 2, after submitting a login, the dialog box would pop up, and just hitting "R" would tell Firefox to remember the password.
In Firefox 3, the 'R' on the "Remember" button is underlined, indicating that hitting the "R" key should work as a keyboard shortcut. But it doesn't work.
This actually makes password handling in FF2 a bit easier to use, at least in my view.