[XeTeX] xelatex,polyglossia,biblatex conflict

[Haines Brown]

I'm migrating from LaTeX to XeLaTeX with TL 2011 on linux system.

  \documentclass[12pt]{article}
  \usepackage{xltxtra}
  \usepackage[backend=biber,style=authoryear,sorting=nyt]{biblatex} 
  \usepackage[style=authoryear,sorting=nyt]{biblatex} 
  \usepackage{xunicode}
  \usepackage{fontspec}
  \usepackage{csquotes}
  %  \usepackage{polyglossia}
  \addbibresource{....bib}
  ...

I would like to use the polyglossia package, but if I add it, I get 
an Undefined control sequence error.

Haines Brown