It seems that the Ring of Loki will not function correctly on higher amounts of ping. I personally have a ping rate of 670+ and when I go to use the omnipresence feature of the ring, The block outline for the selected blocks do not appear. The ring itself works fine though I find myself unable to disable the omnipresence feature. I tested this in single player and the feature worked just fine and the ping rate is the only cause that I could come up with.

It might just not work properly on SMP at all, I need to have a look.