In the June 15, 2007, issue of Science there is this Brevia article:
The Mass of Dwarf Planet Eris
Michael E. Brown and Emily L. Schaller
Science 316, 1585 (2007)
Observations of the orbit of dwarf planet Eris' satellite Dysnomia indicates that Eris has a density similar to Pluto's, but is about 1.27 times as large.
Abstract
Supporting Online Material
Unless one has embargo access, the links above will not be accessible for a couple more hours.
You also might want to keep an eye on Mike Brown's and Emily Schaller's web pages.
See also:
The Dwarf Planet Known as Eris is More Massive than Pluto, New Data Shows
Caltech Media Relations
June 14, 2007