No tropical model is going to be highly accurate seven days out. Most aren't very good beyond three to five days.
There's a lot of variables in play here - a 500mb trough will build in the Eastern US, and these typically cause tropical systems to move poleward (north) then recurve back out over the Atlantic. Timing is everything. Will the trough be there when Isaac gets close to Florida? We'll have a better idea by Saturday.
Isaac still has a lot of structural issues to overcome, the core is poorly defined, there's a lot of dry air entrainment going on. The dry are should mix out within two days, but then you have the mountains of Haiti/DR and Eastern Cuba that will interfere with its structure. Stay tuned.