I attached split the end of a USB wire and attached a switch. When I press the switch, I measure 5V from the port, but am only measuring 33mA. Why am I getting only 33mA instead of 100mA?
There is a decent chance that the computer is detecting a short and limiting the current back.
As pingswept said,
try a 50 ohm resistor
This will stop the computer from thinking there is broken hardware connected.