I've tried 5 different chargers, and none of them gets TP1>TP2 over 4.6 volts. I just plugged in a new charger (found <a href="http://www.amazon.com/Nokia-Original-High-Efficiency-Travel-Charger/dp/B0030T0UOE">here</a>) which gives it 4.4 volts with only SD card attached, but 4.8 with SD card removed.

I tested the power **before** the polyfuse without SD card inserted: 4.94 volts  
Voltage **after polyfuse**: 4.86 volts

So far, so good. Power off Pi, insert SD card, power up Pi and test again:
4.78 volts before polyfuse, 4.39 volts after polyfuse.

I tested the resistance of the F3 polyfuse, it seems to be a constant 1.2 Ohms. Is the fuse bad, or is the SD card somehow using that much power?