There is no "speed dial" that I know of, I just make a short cut to the contact that I want to call on the home screen.
On the Home Screen
Press the Menu Button
Select "Add"
Select "Motorola Widget"
Select "Contact quick tasks"
Select the contact you want
Select what actions you want for the shortcut.
You can also alter the size of the widget if you want.
--Edit--
You can also set a contact as a shortcut, but that won't allow for the "speed dial" affect.