I don't know if this has been already posted: When creating a new message and inserting the subject, the tab-key is not switching the selection/insertion point to the next window, i.e. the Recipients window, but cycles through the As, Name, Adress, Text-Clipping interface elements. One has to select manually the Recipients field or text field by clicking with the mouse.
Cheers, kl. PowerBook Pismo 400 OS X 10.3.3