Ladevale, UA is paying for a large bulk of the $800M costs. It broke ground 16 January 2002 and was the only CIP (Capital Improvement Project) that UA did not suspend this past fall. Miami-Dade County Aviation Authority is also paying a big bulk of it.
Twaneedsnohelp, there is no old Concourse J. Concourse J is a new addition from the ground up. The current "South Concourse" is Concourse H (no Concourse I as not to confuse people with the number 1), which will be home only to SkyTeam by 2006. The current domestic-only terminal is being fitted with 8 international gates for Air France, AeroMexico, and Alitalia.
The new concourse (and very nice, I might add) is Concourse A. It's main tennant is British Airways, but it also houses Aeropostal, LanChile, Air France, TAM, Grupo Taca, Servivensa/Avensa, and a few others. It will be merged into the new oneWorld Concourse (A through E), so it is likely that you will see some carriers move out and others move in (TAM just moved in a few weeks ago).
Concourse A through E: OneWorld. American, British Airways, LanChile, TAM, and other AA partners. Iberia will probably move thier operations from F to A-E. It would be very cool if they could make thier own little "hub within a hub".
Concourse H: SkyTeam and Delta partners. Delta, Comair, Air France, AeroMexico, Alitalia, airJamaica. El Al will probably move here too, and maybe Virgin Atlantic since it will be A380 capable.
Concourse J: Star Alliance and no exceptions. Lufthansa, Air Canada, United, Mexicana, and Varig. Austrian will hopefully be back by then too. bmi, SAS, Spanair, and LOT Polish are all possiblities for MIA service.
Concourse F and G: Most of the leftovers. Northwest/KLM/Continental, USAirways/CC Air, AirTran, Pan Am, Aero Continente, LTU...the list goes on. I have a feeling that the other alliances are not going to be very welcoming when it comes to leasing other carriers gate space unless they are partnered.