T-mobile stores plaintext passwords. They recently invalidated a password I had been using with them for some time because they changed their rules and disallowed special characters (tons of stupid there). They wouldn't have known to do that if the passwords were properly hashed.
edit - just want to state that if they disallow special characters it really is a terrible policy, my point is just that resetting your password isn't proof they are stored in plaintext
Maybe they could have analyzed the plaintext password and stored information about the types of characters it contained and number of characters. Then they salt+hash the plaintext password and store the resulting hash. Now they know a little about the characters and length of the password without knowing the password itself.
Again, I doubt they did this.