Here's supposedly 3 ounces, that's about 85 grams of C4:
http://www.youtube.com/watch?v=d0j_nKYV73I

Considering that the stolen phone would likely be in the delinquent' pocket, a smaller amount would suffice. No point in making our dear phone too much heavier than it already is ;>

If my phone was stolen, I'd cry.
To avoid such, I simply don't trust people at all; I always keep the phone in zip inside-pocket and never let it out of my sight. If I'm in a public place, I usually use a bluetooth handsfree (BH-215, works nicely) to avoid exposing my phone unnecessarily.