Would a Faraday Cage protect something from an EMP? Assume that terrorists manage to detonate an EMP in the middle of the United States. Its range is long enough for the pulse to hit and effectively render useless all unprotected hardware. 
Let's assume that this is a casual buyer and decides just to wrap it in aluminum foil. As for the strength of the pulse, how about lets have be the average strength of the EMP generated by a standard size nuke. 
For example, an E1 kind of EMP could create a pulse with strength of up to 1 MeV (million electron volts)
Background information:
Depending on the size of the gaps in the cage, a Faraday cage can shield an object inside of it from radiation all over the spectrum. If the object is not touching the cage, the object will be sheilded.*
Could a faraday cage protect electronics from an EMP? Could a stronger EMP still damage the electronics within the cage?
 A: It all depends on your construction of the Faraday cage... for a sufficiently well constructed cage (multilayer, continuous, RF gaskets on all seams) the answer is "yes". It's much easier to add another 3 dB of isolation than to double the power of your EMP generating device.
Imagine you have a cage that provides just 3 dB of shielding. If you put that cage inside another cage, you have 6 dB. And so it continues. This is one case where sufficient isolation is indeed possible (assuming you are far enough from the blast that the shield isn't mechanically compromised).
A: If you wrap your electronics in aluminium kitchen foil then the appropriate equation for the electric field transmission factor, that takes into account reflection from the foil and attenuation in the foil is
$$\frac{E_t}{E_i} \simeq 4 \frac{\eta_{\rm Al}}{\eta_0} \exp(-t/\delta) = 0.47 \omega^{-1/2} \exp(-22 \omega^{1/2} t),$$
where $t$ is the foil thickness and $\omega$ is the "frequency" of the EM radiation (see Faraday cage in real life ). The transmitted power fraction would be the square of this.
Typical foil has $t \sim 3\times 10^{-5}$ m and the lowest frequencies have the highest transmission factors. According to this extensive report, an E1 HEMP is less important than lightning strikes for frequencies below 1 MHz. At 1MHz ($\omega \sim 6 \times 10^6$ Hz), the formula above gives a transmission factor of $3\times 10^{-5}$. Given a typical HEMP E-field peak of around 50,000 V/m (same report), then this amount of attenuation is sufficient to reduce the signal to that typical from a strong FM radio station.
So my conclusion is that tinfoil would protect your phone from an EMP. However it is simply not practical or possible to completely enclose all electronic and electronic devices (e.g. they often need cables in or out or some kind of opening, that may leave them vulnerable). 
