What's the one thing your smartphone is missing? That's right, a knife. The Adappt XT iPhone case (currently up for funding on Indiegogo) features a little flip-out tanto-style blade to solve that problem.
At 1.5 inches you won't be using it to fend off ninjas, but if you can get to your opponent before he breaks that bottle, you at least oughta be able to slow him down. And best of all, when the melee's done you can grab your knife--that is, your phone!--and do the decent thing by calling an ambulance for your foe. Then you can use the Maps app to locate the nearest precinct, calculate which direction the cops will be coming from, and disappear in the other direction.
(In all seriousness, I should point out that the case is actually designed to be a multitool and contains a screwdriver, a series of hex-head cutouts and a bottle opener. Check it out here.)
"Give it back, Bernardo. Unless you want to get 'phoned.'"
See also: Iain Sinclair's "Cardsharp" knife