Proceed as follows to reset the service reminder indicator (SRI):
-Ignition position I
-Press and hold in the reset button for the trip odometer
-Ignition position II.
Note! Turn the key to position II within 2 seconds.
-Hold the reset button for the trip meter pressed in until the original value has been reset.
Note! For all vehicles from the 2003 model year a general yellow lamp will light when the reset button must be released.
-Release the reset button for the trip odometer within 4 seconds. The instrument gives an audible signal when resetting has succeeded
-If the trip meter is already reset the reset button should be held in for a minimum of 10 seconds and a maximum of 14 seconds.
Note! If the reset button for the trip odometer is not released within 4 seconds, the service reminder indicator (SRI) is not reset.

A post I read recently said this about the above procedure:
On my '06 S60 it was a little different.. I toyed with the same method that everyone else used for earlier models, but that didn't quite work : you don't see the odo reset back from zero... basically you have to hold it in while in II until the moment that the warning light flashes.. on the 3rd or 4th blink, I released, and that reset the light. Took a bit of messing around for me to figure that one out (and a couple of days of giving up, followed by "argh I want to reset this", and me trying it again differently hehe).

Hope that the above helps you.
Print this whole thing out and take it with you out to your car.
--
1998 S70 T5 Emarald Green Metallic (125K), 2004 V70 2.5T Ruby Red (45K), Previous Owner of Black '94 850 Turbo Wagon (Sold at 140K and miss her). My cars have been running so well lately they've got me worried.