Andrea,
what kind of installation are you using? Gerben's i-installer or fink or have you installed tetex on your own? If you want to find out where different files reside, run this command:
kpsewhere mp-spec.mp
on my system (Gerben's architecture), I get:
/usr/local/teTeX/share/texmf.local/metapost/context/base/mp-spec.mp /usr/local/teTeX/share/texmf.tetex/metapost/context/base/mp-spec.mp
The following comment is only valid for my redistribution: Normally, you would only have the second line (ConTeXt that is part of teTeX 3.0, which is roughly a year out of date). You only get the first line if you have installed my ConTeXt updater package. The texmf.local location is found before the texmf.tetex location, hence, installing the updater overrules what is in teTeX. Uninstalling the ConTeXt updater gets you back to the version in teTeX 3.0. You can set an automatic check for update for i-Packages in i-Installer after which you get a mail message telling you which packages have updated. The ConTeXt i-Package itself is updated automatically on the repositories if Hans releases a new version or a new beta. Which means that you'll hear about any ConTeXt update within 24 hours automatically. The ConTeXt updater is the only i-Package which is updated automatically on the repositories. G