The way the nature style is setup it is easiest to replace the pages with the link (or the volume) but getting the pages and volume is difficult since they are printed at different times. You would have to do a major overhaul of the style. The meat of the solution is
\newbibmacro*{myurl}{\printfield[myurl]{url}}
\DeclareFieldFormat{myurl}{\href{#1}{\printfield{pages}}}
\newbibmacro*{note+pages}{%
\printfield{note}%
\setunit{\bibpagespunct}%
\usebibmacro{myurl}%
\newunit}
combining it to form a complete MWE gives:
\documentclass{article}
\usepackage{filecontents}
\usepackage[style=nature, url=true]{biblatex}
\usepackage{hyperref}
\begin{filecontents}{\jobname.bib}
@article{aksin,
author = {Aks{\i}n, {\"O}zge and T{\"u}rkmen, Hayati and Artok, Levent
and {\k{C}}etinkaya, Bekir and Ni, Chaoying
and B{\"u}y{\"u}kg{\"u}ng{\"o}r, Orhan and {\"O}zkal, Erhan},
indextitle = {Effect of immobilization on catalytic characteristics},
title = {Effect of immobilization on catalytic characteristics of saturated
Pd-N-heterocyclic carbenes in Mizoroki-Heck reactions},
journaltitle = jomch,
volume = {691},
number = {13},
date = {2006},
pages = {3027--3036},
url = {http://www.google.com}
}
\end{filecontents}
\addbibresource{\jobname.bib}
\newbibmacro*{myurl}{\printfield[myurl]{url}}
\DeclareFieldFormat{myurl}{\href{#1}{\printfield{pages}}}
\newbibmacro*{note+pages}{%
\printfield{note}%
\setunit{\bibpagespunct}%
\usebibmacro{myurl}%
\newunit}
\begin{document}
\cite{aksin}
\printbibliography
\end{document}
biblatex, but have you checked 3.11.7 Elctronic Publishing Information, p. 127, of its user manual? You may find suitable solutions there. – nnunes Dec 12 '12 at 01:30