Just got off the phone with the rep.
The book containing superheavies is an EXPANSION and is meant to plug right into the existing rules. They do allow for superheavies and gargantuan creatures.
Each player will have 1 "special slot" (I forget the name) in their FoC for the big nasty things. It must be from your primary detachment. There's supposedly a few equalizers, like a special Warlord chart to roll on if your opponent is taking one, +1 to seize the init, etc.
All the armies that don't currently have a big thing have a FW model that "approved" to take it's place.
Space Marines in all of their forms get............ A Thunderhawk. I guess that lends some weight to the plastic T-hawk rumors.