Once the student signs a letter of intent, the recruiting process is officially closed. Schools are no longer permitted to contact that player.
Criticism of the LOIThe Letter of Intent is intended to effectively end the recruiting process and create a two-way commitment between player and team. In reality, the deal is fairly one-sided.
If the player decides, for whatever reason, that he wants to attend a different school after signing a letter of intent, he must first seek and receive a release from the first institution. This scenario comes up most often when there are coaching changes -- for example, Devin Ebanks asked for a release from his commitment to Indiana after the coach that recruited him, Kelvin Sampson, was forced to resign. Ebanks eventually signed with West Virginia.
Schools don't have to offer releases. On occasion, they'll release players from an LOI under strict conditions, forbidding the player to sign within their conference or follow a coach to his new school.
So, when Kentucky recruit Brandon Knight "signed" with the Wildcats, he refused to sign a LOI, possibly out of fear that coach John Calipari would bolt for an NBA job. The only paper that changed hands was an "aid agreement." It seems likely that other top recruits will take a similar stance in the future.