Hi Tod -

It's not new - it's a form of data validation. You can restrict the values
which can be assigned to a particular cell using the Data Validation dialog.
Click on the Validation menu item in the Data menu. Here you can specify the
validation criteria - which type of value may be entered, min and max value
allowed, etc. If you select "List" from the Allow dropdown, you can then
specify the range of values which may be accepted, by entering the Source.
This is what turns your cell into a dropdown list.

Using the other tabs in the Data Validation dialog, you also have the option
to show an input message which appears when the user selects the cell, and
an error alert message which appears if the user enters an invalid value.

Hope that helps.