Please post the output of
Code:
sudo find / -iname "Helvetica.ttf" -exec ls -l {} \;
It might take a minute or two for this command to complete.

If you have more than one Helvetica.ttf, I recommend removing all but one.
Since each of the directories in /usr/share/fonts/truetype is related to a package, I think it is probably best not to "pollute" those directories by adding files to them that those packages don't expect to be there.

Instead use "gksu nautilus" or
Code:
sudo mkdir /usr/share/fonts/truetype/myfonts
to create a directory for fonts not associated with any package.

Then move Helvetica.ttf into /usr/share/fonts/truetype/myfonts and remove all other copies of Helvetica.ttf that the "sudo find" command found.

Then do a sanity check:
Code:
ls -l /usr/share/fonts/truetype/myfonts/Helvetica.ttf
Make sure the permissions look like this:

Code:
  -rw-r--r--  1 root root 26860 2008-04-13 20:04 Helvetica.ttf
Also run
Code:
ls -ld /usr/share/fonts/truetype/myfonts/
to check that the permissions on the parent directory allow everyone to read the directory:
Code:
drwxr-xr-x 2 root root 4096 2008-04-13 20:04 /usr/share/fonts/truetype/myfonts/
Finally, do a sanity check on Helvetica.ttf:
Code:
gnome-font-viewer /usr/share/fonts/truetype/myfonts/Helvetica.ttf
A screen should pop up showing a rendering of the Helvetica characters.

If all the above does not catch the problem, try this again:
Code:
sudo fc-cache -f -v /usr/share/fonts/truetype/myfonts/
The last command I believe should scan /usr/share/fonts/truetype/myfonts/ and make any fonts it finds accessible to the rest of the system.