Saw an Apple salesman take his house key out of his pocket, grab a bare iPhone, and scratch it quickly all over the glass while pressing down hard.
Not a mark (he didn't scratch the non-glass surfaces, obviously).
It's tough, but it's only glass. Any harder material will scratch it (the key was a brass key I think). Any hard impact will crack it. Things softer than glass (but harder than plastic) will scratch the plastics around the glass.
You'll want to take out some insurance against your daily wear and tear. Get a screen film just to be safe. If you want to protect the plastics for cosmetic reasons, get a film that covers all of it.
Use a case for impact protection. Get a holster/slip case to protect it from impacts while carrying it. If you're likely to bang it while using it, get a permanent covering.
Don't be afraid to combine things. The gel cases might protect it from impacts, but not from any grit that works its way inside the case... if you work all day with sand, metal filings, etc. put a full body skin on the iPhone, then put it in a gel case for impact protection. For total protection, put a full body film on it for scratch protection, then put a gel case on it for protection while using it, and keep it in a hard/holster/slip case when not in use to protect it from you forgetting where you put it and knocking it on the floor or banging your metal toolbox down on it.
Insure it! Get a rider through your home owners insurance before you take it to the construction site. Anything you take to a construction site will eventually have a freak accident, so cover all your bases. Films, skins, and cases are just to limit the times it get's broken to the more freakish accidents instead of a daily occurrence in an abusive environment!
