The accelerometer axes have directions associated with them (just like X, Y and Z).
An acceleration in the positive X direction is positive, in the negative X direction, negative.
Acceleration is a vector. A single acceleration vector may have components along all three sensor axes.
Hi,
If you tip your accelerometer so say the x-axis is down, then you have 1g in that direction, you can do the same with each axis to get the direction sense right.
Tom... 
IIRC LSM303 used in Adafruit's accelerometer+magnetometer sensor has configurable levels for triggering interrupts. This way you don't need to read all values, just wait if interrupt becomes active. If so acceleration was higher than limit and teddy bear is dead.