Make sure you're forcing the division operation to use at least one floating-point argument, or the compiler will compute the INTEGER results of the division (usually ends up 0 or 1), then pass that to asin() as a floating-point number (0.0 or 1.0).
angle = asin(xGForce/1000[glow].0[/glow]);
or
angle = asin([glow]((float)[/glow]xGForce[glow])[/glow]/1000);
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