The 18" wheels were first used on the 2006 model year Limited trim level. They've also since been used on the so-called "Urban" T4R for '08 and '09.
The stock tire size is 265/60-18 which is approximately the same overall height as the 265/65-17 tires used on the earlier 4th gen Limiteds - therefore the speedo would still be accurate. I myself have slightly bigger tires (265/65-18) which provides an extra 1/2 inch more ground clearance and a somewhat beefier look, but it does result in a small speedo error (reads a couple % too slow if I recall correctly).