They gconf key you're looking for is /apps/nautilus/icon_view/text_ellipsis_limit

Basically you just need to set it to [0] and it won't truncate at any zoom level

Here is the full description from gconf if you're curious:

A string specifying how parts of overlong file names should be replaced by ellipses, depending on the zoom level. Each of the list entries is of the form "Zoom Level:Integer".

For each specified zoom level, if the given integer is larger than 0, the file name will not exceed the given number of lines. If the integer is 0 or smaller, no limit is imposed on the specified zoom level. A default entry of the form "Integer" without any specified zoom level is also allowed. It defines the maximum number of lines for all other zoom levels.

Examples: 0 - always display overlong file names; 3 - shorten file names if they exceed three lines; smallest:5,smaller:4,0 - shorten file names if they exceed five lines for zoom level "smallest". Shorten file names if they exceed four lines for zoom level "smaller". Do not shorten file names for other zoom levels. Available zoom levels: smallest (33%), smaller (50%), small (66%), standard (100%), large (150%), larger (200%), largest (400%)

Edit: This is now obsolete, an extension has been written for this: https://extensions.gnome.org/extension/153/nothingtodo/

There's no configuration value for this, it's hard-coded in at the moment, but it's easy enough to change:

gksudo gedit /usr/share/gnome-shell/js/ui/panel.js

Find the line that reads:

this._dateMenu = new DateMenu.DateMenuButton({ showEvents: true });

And simply change true to false

Then restart GNOME Shell by pressing Alt-F2 and entering r

A useful resource for reference (see components section): http://blog.fpmurphy.com/2011/05/more-gnome-shell-customization.html

Edit: For anyone who's interested, here's the end result: