Apparently there are some nice similarities between Tango and Oxygen... both apply this outline to icons, and they seem to use almost the same color palette (Oxygen's palette has a few more colors). A very Firefox-ish thing. One interesting thing in Oxygen is that they have a "normal" color palette, and a "vibrant" color palette. Maybe that's an interesting concept.
Oh well, i just hope these folks shake hands and would create a unified (in one way or another) icon set for the free desktop.