I got an email from a mi.yodeya user (Dave) who says that he can't access his account here. He registered it on mi.yodeya using the email/password method. Is there a general issue for such accounts?
If you were an unregistered user (ie, your account just lives in your browser via a cookie), due to the domain change, you'll need to recover your account. You can do this via the login page:
1) Go to https://judaism.stackexchange.com/users/login
2) Click the recover link
This will prompt you for the email account you used on your account here. An email will be sent to you that contains a link that will recover your account.
Once you do this, you can continue as an unregistered account, or you can register and associate an OpenID account so that your account will persist beyond a browser cookie.