iphone: floats cast to unsigned ints get set to 0 if they are negative?

This is to be expected - casting a negative float to an unsigned int results in undefined behaviour (UB). If you want the value to wraparound (which is also UB, BTW), then you would need to cast to a (signed) int first and then to unsigned int. Ideally you should not rely on UB at all and find a better way of doing what you need to do.

§6.3.1.4 of the C standard:

When a finite value of real floating type is converted to an integer type other than _Bool, the fractional part is discarded (i.e., the value is truncated toward zero). If the value of the integral part cannot be represented by the integer type, the behavior is undefined.

So like Paul R said, this is undefined behavior.