TileMill 0.2.0 is now available for download with several new features and bugfixes. The most noteworthy new feature is support for authoring interactive maps, like the one below. Interactivity authoring allows you take data from a geographic datasource and present it as tooltips or popups on the map.
Mouse over the countries on this map to see the interactive tooltips.
To add interactivity to a project, open the project and click on the "Interactivity" tab under "Settings".
Choose the layer that you want to be interactive.
Select the name of the field that is unique across all features in the layer. It can be any field as long as each feature has a unique value for it.
Provide your formatters. You can add text, HTML, CSS, and replacement tokens (see #4). By default, "Teaser" will display in tooltips on hover and "Full" will display on click.
Use these replacement tokens in a formatter, and they will be replaced with the data for each feature.
Click "Save" and close the settings window. The map will now reflect your interactive settings.