Quoted from a network security mailing-list I am subscribed to:

Last time [we] sent out a warning email along the lines of:

We never ask for your username and password. If you get an email that looks like:

"There is an issue with your account. Please reply with your username and password and we will rectify it"

You should never reply to these messages with your details.

50 people replied with their usernames and passwords.