DraftExpress' Jonathan Givony has a fantastic article up that explains the NCAA and NBA deadlines and regulations governing this year's NBA Draft.
Essentially, what Givony has deduced is this.
(1) The NCAA has mandated that any underclassman who declares for the Draft has until April 10 (the day prior to the late signing period) to withdraw from the draft.
(2) The NBA has set the deadline for underclassmen to enter the Draft as April 29.
(3) Prior to the official release of the early entrant list in early May, the only contact between a prospective player and an NBA team is communication between the player's college head coach (i.e., Rick Barnes) and principal NBA team executive (i.e., Oklahoma City's Sam Presti). The player, his family members, or any other third parties must not contact the team at any point prior. Further, NBA coaching staffs may not get involved, only the front office lead.
(4) The NBA provides a service in which the NBA Undergraduate Advisory Committee will offer a projected Draft status to any undergraduate applicant. The Committee sends out these responses on April 6, prior to the NCAA withdrawal deadline of April 10. The application for a response is not a declaration of intent to enter the Draft, and as Givony points out, is not always an accurate representation of where a player will end up going in the Draft.
(5) Essentially, a prospective player can gather only limited information prior to both the NCAA withdrawal deadline (April 10) and the NBA eligibility deadline (April 29). That information can be gathered whether a player has declared for the draft or not.
(6) Without the opportunity to work out for prospective teams after the official release of the early entrant list (as had been the norm in years past), there is essentially no ability to "test the waters." If you're in, you might as well stay in.
(7) There is little incentive to declare for the NBA Draft prior to April 10. Any player that has declared for the Draft prior to April 10 has until April 10 to decide whether to stay in the Draft or not. Any player that has declared for the Draft after April 10 is bound by the NCAA to stay in the Draft. However, any player that has not declared for the Draft after April 10 is still NCAA eligible until officially declaring. If you're not a surefire pick like Kentucky's Anthony Davis, why not just wait and declare on April 29?
(8) For Longhorns fans, that means that if J'Covan Brown and/or Myck Kabongo declare prior to April 10, there's less than a puncher's chance he will withdraw. But, it would actually be self-defeating for Brown or Kabongo to declare prior to April 29, as there is no difference between officially declaring and unofficially thinking about it, except for the rule that a player who officially declares after April 10 is no longer NCAA eligible.
(9) That also means it may be a longer April than we had previously thought.