Similarly to what happened in earlier versions of Ubuntu, the default tooltip colors of the Ambiance theme (Ubuntu default) results in unreadable Eclipse tooltips (black foreground text on black background).
In earlier versions of Ubuntu you could edit the Ambiance theme via Gnome control panel and change the tooltip colors. Unluckily, the new "Appearance" settings dialog of Ubuntu 12.04 allows you to select the theme but you cannot tweak its colors anymore.
So here is how you can fix the problem.
sudo apt-get install gnome-color-chooserrun the tool and switch to the Specific tab. Activate the Tooltip group and set your tooltip colors (I use black foreground on white background). Then, at the bottom of the dialog select the value Theme Default for the profile combo and press apply. The next time you start Eclipse it will honour your tooltip colors.
For those who may be interested, gnome-color-chooser will change your ~/.gtkrc-2.0 file.