It's not just domain registrars, I reset the password of my basecamphq.com account (which stores very confidential project information) last week, and received this email:
Hi -name-,
Can't remember your password? Don't worry about it — it happens. We can help.
Username: -username-
Password: -password in plain text-
Please keep your password safe to prevent unauthorized access.
It blows my mind that even 37signals falls for this trap. There should be a website showing a blacklist of services that store passwords plaintext.