More Mac 101, TUAW's series for beginners. My mom called me up last week with this question about Mail: "Where did my send button go?"
Turns out she had mistakenly clicked the white chiclet (pictured) in the upper-right hand corner of her message's compose window, hiding her toolbar. When she clicked it once, Mail remembered her settings and hid the toolbar for other message windows, too.
So, just clicking that chiclet showed the toolbar again, and brought her send button back. Magic.
Also, you can hold down command and click the same chiclet to show and hide icons and text labels in the toolbar. Repeatedly clicking with the command key held down cycles through all your options.