This is from memory so bear with me...
Start = pause, can choose to exit race or change the screen settings.
Select = pause on/off
X = select item or choose between 3 different views (bumper, chase cam, and full panoramic camera views) of various cars
O = call in for pit stop. After pit, you can use the D-pad and X to toggle various choices (which tires, how much fuel {if any}) etc.
Triangle: go into "overtake" mode (helps to tell your driver when to pass and when not to pass)
^...that one was edited from Fabmaru's post below, thanks. 👍
[]...not used?
D pad left & right = toggles thru various menu items & speed ratings from 1 to 5
D pad up & down: switches from one car to the next
R1 = switch to Race Monitor and back to race
hold L1 + D pad left or right= switches the speed of the race from X1 to X2 to X3 (damn, we need this in GT5!). You gotta hold L1 while you do this, though.
R2 & L2: not used (I think)
R2 = switches HUD on and off
L2 = not used
Analog sticks R3 & L3: not used (I think)