<p>Is there a way to prompt the user for input during a NAnt build? I want to execute a command that takes a password, but I don't want to put the password into the build script.</p>

A solution I have used many times is to have a
local
config file containing such things as passwords, connection strings etc. that are specific to each developer. The NAnt build script will include these settings when building.
The local config file does not exist in the version control system so passwords are not exposed. The first time a developer checks out a code base and tries to build he has to create this config file. To make it easy for him, there could be a template file available such as my.config.template containing all the properties that can be customized.