This is quite a late response, but here is how I have adapted tufte-latex to be used with biblatex and xelatex.
As I use a lot of margin notes, margin figures, etc but I also cite
references within sidenotes. This became cumbersome and messy, so I had
to alter the default citations in Tufte to be a bit more compact.
Basically a citation here is setup to be "Author, [bibliography
number]" instead of the usual tufte version.
With this setup I
use \textcite{YourReferenceLabel} when referencing something inside a
\sidenote{}, which will print the citation inline in the sidenote text. Then
in the main text I use \autocite{YourReferenceLabel} which will print
the reference as a sidenote.
\documentclass[nobib]{tufte-book}%
%XeLatex specifics
\usepackage{fontspec}
\usepackage{xunicode,xltxtra}
%% font definitions
\defaultfontfeatures{Mapping=tex-text}
\setromanfont{Your Roman Font Name}[Mapping=tex-text]
\setsansfont{Your Sans Font Name}[Scale=MatchLowercase, Mapping=tex-text]
\setmonofont{Your Mono Font Name}[Scale=MatchLowercase]
%Tufte-Latex Spacing Work around for XeLaTeX
\renewcommand{\allcapsspacing}[1]{{\addfontfeature{LetterSpace=20.0}#1}}
\renewcommand{\smallcapsspacing}[1]{{\addfontfeature{LetterSpace=5.0}#1}}
\renewcommand{\textsc}[1]{\smallcapsspacing{\textsmallcaps{#1}}}
\renewcommand{\smallcaps}[1]{\smallcapsspacing{\scshape\MakeTextLowercase{#1}}}
%BibLatex Setup
\usepackage[
natbib=true,
bibstyle=numeric,
block=nbpar,
citetracker=true,
citestyle=numeric,
backend=biber]{biblatex}
%% Add bibliography files
\addbibresource{YourBibliographyFile.bib}
%Recommended for Biblatex/Biber
\usepackage[autostyle]{csquotes}
%% Commands for cites to work with biblatex
\usepackage{hyphenat}
\usepackage{hyperref}
\hypersetup{%
unicode=true, %non-Latin chars
colorlinks=true, %false: boxed links, true=coloured links
linkcolor=blue, % color of internal links
citecolor=green, % color of links to bibliography
filecolor=magenta, % color of file links
urlcolor=red % color of external links
}%
%%% End Preamble
-Steve