Well, as the link to the discussion in 406 points out... the northward trend should have been expected. We have seen the same thing with the prior storms this year, and there is no way the ridge was going to hold quite as strong as it was for a week straight. The main ridge is centered over the Atlantic, and we have continually seen it pulse in and out of the GOM for the past few weeks. It was strong recently, and looks to be pulsing out now (just in time for the storm to arrive). The affects will still be felt enough to keep the storm from hooking, but the weakness looks to be just enough to keep it on a fairly steady path.
People talking cat 5 are getting ahead of themselves, but this should be a sizeable storm... the worst option here is if it shoots through the Yucatan channel, because we will see no break in the waves it's building. A long track storm, without hitting some sort of landmass prior to landfall is going to have a heck of a surge.