Citing the two papers as "Openshaw et al (1999a)" and "Openshaw et al (1999b)" implies that those papers were written by the same co-authors, while in fact one author (John Davy) was part of the second, but not of the first author team. (Herbert's use of the alpha style avoids this problem.) Using author-year-styles, I would cite the papers as "Openshaw, Turton et al (1999)" and "Openshaw, Turton, Macgill et al (1999)". I'm not aware of a bibliography style that will do so automatically (disambiguation of author lists is on the roadmap for future versions of biblatex). Using natbib, you could define
\defcitealias{key1}{Oppenshaw, Turton et al (1999)}
\defcitealias{key2}{Oppenshaw, Turton, Macgill et al (1999)}
and write
\citetalias{key1,key2}
in the text, but this solution is far from perfect (\citepalias would result in double parentheses).
UPDATE: Disambiguation of author names and name lists was implemented in biblatex v1.4, released on March 31st, 2011. See section 4.11.4 of the biblatex manual for details. Have a look at the following example with the package option uniquelist=true:
\documentclass{article}
\usepackage[style=authoryear,maxnames=1,uniquelist=true]{biblatex}
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@article{Openshaw1999a,
author = {Openshaw, Stan and Turton, Ian and Macgill, James},
title = {Using the Geographical Analysis Machine to Analyze Limiting Long-term Illness},
journaltitle = {Geographical and Environmental Modelling},
year = {1999},
volume = {3.1},
pages = {83-99},
owner = {ijt1},
timestamp = {2010.09.27},
}
@incollection{citeulike:8468480,
author = {Openshaw, Stan and Turton, Ian and Macgill, James and Davy, John},
title = {{Putting the Geographical Analysis Machine on the Internet}},
booktitle = {Innovations in GIS 6},
publisher = {Taylor and Francis},
year = {1999},
editor = {Gittings, Bruce},
chapter = {10},
pages = {121--132},
location = {London},
}
\end{filecontents}
\addbibresource{\jobname.bib}
\nocite{*}
\begin{document}
\printbibliography
\end{document}

For comparison see the output of the same example with uniquelist=false:

natbibstyle such asplainnatorabbrvnat. What bibliography style are you using? – Olof Jan 19 '11 at 23:30