The Tholians and the RSE. There's no love lost between RSE and Federation esp. in light of the cozying up to the IRS. Oh, and the Tzenkethi are furious over the death of one of their citizens, a child "at the hands" of the Federation (regardless of how events actually played out). The Kinshaya are at war with the Klingons, and the Federation wouldn't want the Khitomer Accords to fall through again. Apart from the Gorn, it looks like a pretty hostile Pact to me. Maybe not hostile enough to declare open war, but enough to be on a Cold War footing.
I mean it's all very well postulating that the Typhon Pact isnt going to be an enemy, just another political player etc.. however that seems to be nothing more than conjecture, as everything actually seen in any books paints them as hostile.
As soon as they thought they had the backing of the pact, the Kinshaya attacked two Klingon worlds. Through proxy agents, memebers of the pact tried to destabilise the Federation by sabotaging mining efforts and antagonising one of the planets (I forget it's name, begins with "Z" I think...) to temporary stop taking refugees and threaten withdrawl from the Federation itself, the Tholian ambassador out and out said that they were only joining to spite the Federation and how the Federation and Klingons are now "surrounded by a heavily armed power".
Even the blurbs for the upcoming books paint them in a hostile manner. "2010: the Typhon Pact awakes", Titan is going up against the Gorn as they attempt to solidy there place in the Pact, the Aventine has to place Federation agents "behind Breen lines", DS9 has to deal with the RSE (Those well know, peace loving friends of the Federation) trying to "establish themselves as the rightful leaders of the Pact", by "any means necessary", and the TNG book mentions how "The Typhon Pact will show their strength by daring to reach into the heart of the Federation to raise their standard as the pre-eminent power", etc..
There might not be out and out war, but the intentions of the Pact seem quite clear at the moment. And those intentions do not point towards peaceful co-existence.