On one hand, the US5881 Hall Effect sensor (sold on AdaFruit WebShop) does not appears to work with 3.3 volts as power supply (cfr tech sheet).
On the other hand, Raspeberry GPIO are not 5v Friendly.
The US5881 usually use a pull-up resistor of 10K to +5v, the US5881 will pull-down the voltage to GND when activated.
See http://mchobby.be/wiki/images/thumb/a/a ... ontage.jpg for schematic.
For Raspberry PI, I would propose to build a resistor bridge with an additional 20K resistor between the Output Pin of US5881 and GND.
By acting like that, Output voltage should be 3.3v (when sensor not activated) or 0v (when the activated sensor pulls down the voltage to GND).
Am I right?