That is based on an unofficial ranking that is going around Facebook - the official IRB rankings were last updated on the 5th August and I believe will be updated again this monday coming after all the weekend matches. NZ are top on 91.54 points with Wales 2nd on 89.96.
This weekend, NZ have lost to Australia [26-47], Ireland beat italy [29-10], South Africa beat Argentina [46-13] and Wales play England today, so if Wales beat England they will go top.
Edit
Sky TV have just told me that if Wales beat England OR draw, they will officially go top.
Additional edit
England win 33-19, probably keeps NZ top.