Is it possible to install the Helvetica font on linux?
The simple answer is to buy the font right from the official distributor, Linotype. They sell individual variants for $26, or packages for a variety of prices depending on what they include.
With the basic license (which comes with any Linotype font), you're free to do whatever you want with any documents you create using the font... personal use, professional use, whatever, it doesn't matter. The font file itself may be installed on up to 5 computers.
Technology-wise, the font is available in PostScript, TrueType, and OpenType variants; all of these are useable on Linux if you have the right software installed.
EDIT: As far as the dirty details of licensing issues: Fonts have been legally classified as computer software. You're not buying the actual image of the font, just the code that creates the image. (This is why other companies can create fonts that look so incredibly like Helvetica without infringing on the copyright.) Like Microsoft Word, once you buy it, any content you create with it is entirely your own.
The license agreement, by allowing installation on multiple computers, represents a sort of voluntary waiver (by the author) of protections generally guaranteed by copyright, rather than an addition of new restrictions, and so would likely hold up in a court of law. In common language, the license says, "Even though this work is covered by copyright, we're gonna be nice and let you make some extra copies anyway as long as you stick to our terms."
That just leaves the question of fair use, which is a big one. I'm not aware that any court has established any sort of fair use doctrine for fonts. Copyright cases in other media have established that in certain cases, making copies of a protected work for personal, private, noncommercial use is legal. As far as I know, this is a pretty huge Grey Area as far as fonts are concerned. If you choose to go this route, you might have a strong case that it's perfectly legal, on the very low chance that anybody actually wanted to try to sue you for it. (The question of where on the internet to find a free copy of Helvetica is beyond the scope of this answer coughpiratebaycough.)
Copy helvetica.dfont from your Mac and use Fondu to convert it to ttf and then move it to your fonts folder:
http://grasshopperpebbles.com/ubuntu/ubuntu-convert-mac-dfont-files-into-ttf-using-fondu/
Summary:
sudo apt-get install fondu- Navigate to where you copied the .dfont file
fondu Helvetica.dfont