To understand concerns in the ARIN region, I'd recommend that folks read the actual policy text and staff assessment at <https://www.arin.net/policy/proposals/2009_3.html>
Because this global policy results in significant departure from RFC2050 need-based allocation (with one equal allocation unit per RIR each 6 month period even if less than regional demand), there are challenges of equity that result from mandating the return to the IANA.
return is input to the pool which is quite different from [need based] allocation, which is output from the pool
The reality is that returned address space has been going back to IANA and will continue to do under current practices
problem is that arin's unique rewording rather blatantly hedges that for the future. randy