Hey Mike, I'm sure you figured this out already, but when you first create the attribute type a number with the inch mark like 0" or 0' in the value box and hit enter. This will set the unit for that attribute to inches. This only works for the first time a value is entered. After that first initial value, you will need to use the attribute info dialog to make changes.

In addition, you can use 0 to set it to text (not "Default: Text") and 0cm to set it to centimeters.

If anyone knows a shortcut for setting the unit to Decimal Number, I would love to hear it!