I don’t often batch import layered files (PSD, PNG) into Flash or even copy graphics into it.  But what I DO find really handy is copying text between the apps.  This worked in CS3 but it’s gotten better (more accurate) in CS4.  You can copy a whole group in and the font, size, and color are all maintained (or close enough, you usually have to nudge line height).  This saves tons of time when you’re populating a whole FLA from a text heavy design while keeping the text editable for the inevitable copy change.  Notice in this video how I can bring in multiple text boxes at a time.