The latest rules, which take effect for players entering college after August 1, 2011, do affect D1 and D2 eligibility.
Let's take D2 (14.2.4) first, because it is simpler and less onerous. In D2, a player won't lose a season of eligibility if he delays enrolling full-time for one calendar year. For example, if he graduates from high school in May 2011, the one year period lasts until May 2012. If he enrolls full time at the next available regular academic year opportunity (probably September 2012), he's OK. In general, he needs to enroll full time at the next opportunity (except summer term) after one year has elapsed.
In D1, the new rule is 14.2.3. As I understand it, a player who graduates in May 2011, must enroll full time by May 2012, or be subject to the following:
If he plays in a game after May 2011, and before full time enrollment, he loses a season of eligibility and has to serve a year in residence before he can compete.
So I think this complicates doing a post-graduate year for students who end up in D1. Either the player needs to enroll full-time in a college for the summer session (I'm not sure if this is sufficient, but probably), or he can not compete in baseball until he does enroll full-time. Otherwise he pays a heavy eligibility price.
See also
this thread.