Josh,
Maptips from the MXD/MSD are not available in Flex. There is a great amount of overhead that is involved in having an attribute shown for a particular geometry when the mouse is over it and a desktop client is meant to handle this, but a web client like flex is not intended to have this level of complexity. Now before someone else pipes up and say that yes you can have maptips in flex let me reiterate what I am saying. Maptips are possible in flex with some pretty intensive development code, but the web client was not intended to be an online ArcMap and there for you should steer away from this type of development so that your users can have a fast web experience.