Just read this thread on another site (mods feel free to remove it if you wish):
Vodafone NOW unlocking phones for free. - HotUKDeals Forum
Details are a bit sketch for PAYG and i think it depends on the operator you speak to, but it sounds like they will unlock Voda locked phones for free, and all Voda Contract phones will now be SIM free.
It's worth reading the official Voda thread here:
Network Unlock Code ( Nuc ) - Vodafone eForum
Let us know how you get on (i've emailed for a K800i code)