IMO Castro is reacting to rumblings at home and abroad, some probably prompted by the war on his dictator-in-law Saddam, and this is only the beginning of a wider purge in the Cuban government. The question will be, will Castro’s still-iron-but-aging-grip be enough to hold on to power? Or will there be a military or popular uprising? My guess is that Castro will open his perennial escape valve and make some pronouncement about allowing anyone who wants to, to leave Cuba. However, that might not fly so well with the current US administration, so who knows.
It is no secret that Castro is taking advantage of world attention being focused elsewhere to carry out his repression. It has happened in the past. Contrary to what many think, I think many countries are getting tougher for two reasons: Big problems elsewhere mean they get less attention, and also because a higher level of tension seems to justify getting tough.