A person's username is included in the email.
QUOTE
<#NAME#>,
This email has been sent from <#BOARD_ADDRESS#>.
You have received this email because this email address
was used during registration for our forums.
......
The password is hashed before the email is sent, and there is no way for IPB to retrieve it once it has been hashed. The password could in theory be included with this email, but I could see this as being a potential security risk myself...packet sniffers, or if an email bounces and is logged in the ACP, etc. Besides, the "Lost Password" tool is available to users to reset their passwords already..