As per the documentation:
Use the TextField.defaultTextFormat property to apply formatting BEFORE you add text to the TextField, and the setTextFormat() method to add formatting AFTER you add text to the TextField.
The TextFormat properties are null by default because if you don't provide values for the properties, Flash Player uses its own default formatting.
Source: http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/flash/text/TextFormat.html