I only have xdvi installed (linux system). However, iTeXMac is incorrectly detected as installed because the file /usr/bin/open exists on my system (man page says: “NAME openvt - start a program on a new virtual terminal (VT).”, see http://packages.debian.org/squeeze/console-tools\).
TeXlipse should ideally detect that this is not the right program and display it as not installed in the viewer settings pane. It would also make error messages clearer for the user, e.g. when trying to preview a PDF file TeXlipse could realise that there's no PDF viewer installed and inform the user, instead of trying to start what it thinks to be iTeXMac.
Also, trying to start "open (parameters)" when it's not iTeXMac could also have undesired side effects. Luckily "open" does not understand parameter -a and fails, but it could be worse.