Originally Posted by JDiver
Sorry - the bad news is you will have to pass through imigration, recover your checked baggage and proceed through U S Customs. Then you will have to recheck your bag(s), exit and pass through security to depart the US again. Unfortunately, the USA did away with transit paths and lounges. 90 minutes can be very tight, IMO. Depending on time, the immigration queues can be dreadfully long; you will be able to save some time if you travel with carryons only.
This is no longer true!!! The procedures changed a few months ago, and sometimes transit is possible without entering the US. The procedures vary by airline, so you have to check the specific flights, but MIA has implemented this a couple of months ago for some flights. Presently several airports have such arrangements in place (for example, LAX at T2 for NZ AKL-LAX-LHR, allowing transit without entering the US).
In general 90 minutes is plenty for international connections in MIA, despite all the horror stories (it usually is not bad, and sometimes quite pleasant, for example in the new D terminal which has great immigration/customs facilities with over 30 lanes). A lot has changed at MIA in the past few months, it is a different airport from even last summer (at least the AA part is improved).