Built-in drop down menu support is something we’re working on right now for Thematic 0.9 (one of the many changes you’ll likely be hearing about pretty soon). In the meantime, the always resourceful Cozmoslabs has released a nifty little Child Theme for Thematic called PageDrop that adds drop down menus to the current Thematic style. I’m sure this will be very popular. Check it out.
ThemeShaper 