Yes, it’s possible. TFT and Mojo both told me it’s not, but that’s untrue.
I started a thread on mtbr a while back if you want the full low down:
http://forums.mtbr.com/shocks-suspension/2011-van-180-travel-reduction-using-van-160-spring-709405.html
In short:
The Van 160 spring is 55mm shorter than the 180 spring. This means you can drop the fork down to as little as 125mm (180-55) without cutting the spring, just using spacers.
I set mine to 135mm using a 20mm and 25mm spacer on top of the top out spring on the plunger shaft. A 10mm spacer was needed on top of the main coil to make up the required length (55mm – 45mm travel reduction = 10mm spacer needed, otherwise the main spring will just be rattling around).