Why are you wanting to hash floating point values? For the same reason that comparing floating point values for equality has a number of pitfalls, hashing them can have similar (negative) consequences.
However given that you really do want to do this, I suspect that the boost algorithm is complicated because when you take into account denormalized numbers different bit patterns can represent the same number (and should probably have the same hash). In IEEE 754 there are also both positive and negative 0 values that compare equal but have different bit patterns.
This probably wouldn't come up in the hashing if it wouldn't have come up otherwise in your algorithm but you still need to take care about signaling NaN values.
Additionally what would be the meaning of hashing +/- infinity and/or NaN? Specifically NaN can have many representations, should they all result in the same hash? Infinity seems to have just two representations so it seems like it would work out ok.