There are tons of dates available all through 2014 and even into 2014. At this point it looks like American and US Airways are the ones offering these fares, but I wouldn’t be surprised to see DL or UA match in part. Some of the departure cities include:
$300 RT to the Caribbean, especially to some of the more remote islands like St. Lucia or St. Maarten is definitely low.
You can see more details on The Flight Deal.
I don’t expect a fare sale like this to last very long, and you likely won’t be able to book next year’s spring break trip, as it expires in March, 2105. But, there are plenty of great opportunities for a long weekend or a trip over the summer if you’re looking to be in the Caribbean.