From radar: <rdar://problem/4063719> If your primary language is Hebrew (or Arabic) and you have the Hebrew (or Arabic) localizations installed, when you open a new document in TextEdit, the writing direction is automatically set to be right-to-left. The same should be true of new compose windows in Mail. For replies, forwards and the like, perhaps this should depend on the encoding of the quoted content.
This is fixed in Leopard Mail.