MacTeX installs a data structure by Gerben Wierda and Jerome Laurens to support multiple TeX distributions on a machine. Users who installed last year's MacTeX or Gerben Wierda's TeX from several months ago will discover that the old TeX has not been altered by MacTeX. Instead a new preference pane is available in Apple's System Preferences; this "TeX Distributions" pane lists available distributions on your machine. After installation of MacTeX-2007, it might list "gwTeX-2003-2005" and "TeXLive-2007", where gwTeX-2003-2005 is the distribution contained in last year's MacTeX. If you have installed Gerben Wierda's most recent distribution, it will also list "gwTeX". The currently active distribution will be selected in the pane. Select another distribution to make it active.