SD cards (the full-size ones) can use up to 330mW of power (100mA at 3.3V) - The Pi is known for having difficulty with certain high-speed (and thus high-power) cards, so I believe it is less than 330mW - but how much less?

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The schematics @ raspberrypi.org (I checked Revision 2.1 for type B) do not suggest any deliberate measures to limit the current to the SD card.

Vdd of the SD card is directly connected to the 3.3V rail which leads to the respective voltage regulator. There are of course some possibilities that might limit the current:

  1. Obviously the Pi cannot distribute more power than available from the supply.
  2. The design load of the 3.3V regulator is limited to about 1000mA* and the two other voltage regulators (2.5V and 1.8V) are daisy chained to it. So essentially all load is coming through here. This includes GPU, CPU, RAM, NIC, HDMI, ... and the Pi specific peripherals such as the camera. The documentation is a little vague as to how much exactly they consume.
  3. From schematics alone it is not possible to quantify the resistance of the power line on the circuit board from the regulator to the SD card. However a prudent design would use power planes or a sufficient width of the pcb tracks.

With respect to the second point however an increased load (e.g. heavy use of GPIO to the max current and connecting the camera module) should lead to increased amount of failures with SD card access. Are there any reports about this behaviour?

*) The used regulator NCP1117 is specified with an output current in excess of 1.0A. The raspberry's schematics denote however a heat sink copper area on the board according the a power dissipation design load of 800mW (unknown design temperature however). Given the necessary dropout voltage (supply voltage - 3.3V = 1.7V) this thermal load would be hit at about 470mA. Woo, that seems quite low but then again it is just a rough estimate according to a annotation in the schematics.

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