Imp mangle has almost no situational use. Survival of the Fittest is only useful in hybrid builds. I was able to take virtually every optional talent that would have any reasonable use for cats - and point of fact, with the tree the way it is currently you have to take non-cat talents early on to get anywhere. And that's at level 80.
Nom Nom Nom requires no feral aggression; you use FB instead of rip when your target is at 25% or less. It's a huge win of DPS at that point regardless; essentially it makes rip do 20-25k damage on initial application.
I can't determine what you're referring to when you say "you have to take non-cat talents... to get anywhere."
I think you took me too literally when you say that FA isn't required for NNN. To clarify, I was pointing out that with NNN, we will be using FB quite a bit more, hence FA becomes more valuable. However, this point is probably moot because there seems to be no reason not to take FA in the new talent trees. FA is a no brainer.
One preliminary question I have is to what extent Ravage, with Imp Feral Charge (IFC), will be useful in raids. When movement is required, it might be optimal to charge either out or back in, and use 2-4 ravages rather than the normal rotation.
The second question, as I noted, is to weed out which utility talents are best. Imp Mangle will really only be useful if they throw another meaningful Kologarn fight at us, which seems unlikely. We skip it now, and it seems safe to continue doing that in Cataclysm. Brutal Impact is probably the most important utility talent for cat. Other than that, we have some options, which I believe was the stated design goal. I'm thinking something like this at the present time:
WoW Talent Calculator - Sigrie