You could send an e-mail that has a single use one-click autologin deal that takes them to their account preferences so they can do whatever they might want there.

EDIT: To address the original question, if I got a plaintext back from a web site I know it's bad, but I don't reuse passwords, so for me it is not the end of the world (assuming it's not my bank or something!) Hopefully this is generally true of more technical users.