Do you know that now in flash you can render text as html. See the below example.

Mixing and matching HTML tags allows you to create more sophisticated-looking text

Flash supports the onlye a few HTML tag

To render a text as html in a textfield or a dynamic text field.
You have to use the text field property "htmlText"

eg:
youTextFieldname.htmlText = "<b>Your text content</b>";

This will display the text "your text content" in bold in the text field