From Jeff Master's blog on wunderground (this also answers my question about the increase in size):

"When hurricanes undergo a major eyewall replacement cycle like Emily's current one, they often take a jog in their forward motion. This has critical implications for Jamaica and the Cayman Islands. A substantial jog to the north in the next few hours would bring the core of the hurricane directly over Jamaica tomorrow. When Hurricane Dennis approached Cuba last week, the storm did an eyewall replacement cycle and wobbled to the north. However, there is no way to foretell what Emily's current reorganization will do to its track.

Another consequence of Emily's reorganization is that the storm appears to be expanding in size. Emily was a compact spiral of clouds last night, with it spiral bands not quite stretching north and south to span the Caribbean. Now, the storm fills the Caribbean more completely. This will also have important and unforecastable consequences on Emily's track, as a larger storm will respond differently to upper-level steering currents.

The computer models are tightly clustered around a track bringing Emily across Jamaica and the Cayman Islands, then into the Yucatan Channel between Mexico and Cuba. I've mentioned in the past that when the models tightly cluster like this, it's usually a pretty sure bet that the storm will follow the suggested path. However, the models have been tightly clusted for about 12 hours now, but Emily has not budged from its steady path south of the predicted model path. This may be a case where all the models are using the same bad data as input, and thus generating an incorrect forecast. Thus, NHC anticipates Emily will stay south of Jamaica and the Caymans, and hit Mexico's Yucatan Peninsula instead. I don't think we'll know the answer until late this afternoon at the earliest, when Emily finishes its reorganization and resumes intensifying again."

Wunderground has also started a blog from someone on the Cayman Islands, which should be interesting to read during the next day or so.