I bought a 4-yr old IS about 2 months ago, had severe rattle which was fixed a week after I took delivery.
Mainly from the left and right front door, dash squeek, and sunroof.
The roof had been replaced with a new one about 2-yrs ago, and probably will be there for the life of the car.
Dash was taken apart, there was a TSB in the states on the 05-06 model.
To date it's partially resolve, and will probably get this re-look at again next month for the schedule service.
Apparently this is pretty normal with toyota / lexus.
Sam