UPDATE:  Moving to the front as a reminder (and I've had some reports, from faculty and students, of schools trying to force earlier decisions)

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A philosopher at a PhD program writes:  "For the first time I'm involved with grad admissions, and we're already hearing from our candidates that we have waitlisted for fellowships that other schools are trying to impose deadlines for their fellowship offers in advance of the April 15 standard.  (One offender is trying to impose a March 5 deadline.)  Would you consider using your blog to let your grad school applicant readers know that they cannot be railroaded into a decision earlier than the April 15 standard?"

The American Philosophical Association's official position on offers of admission and aid is here:

Students are under no obligation to respond to offers of financial support prior to April 15; earlier deadlines for acceptance of such offers violate the intent of this Resolution. In those instances in which a student accepts an offer before April 15, and subsequently desires to withdraw that acceptance, the student may submit in writing a resignation of the appointment at any time through April 15. However, an acceptance given or left in force after April 15 commits the student not to accept another offer without first obtaining a written release from the institution to which a commitment has been made. Similarly, an offer by an institution after April 15 is conditional on presentation by the student of the written release from any previously accepted offer. It is further agreed by the institutions and organizations subscribing to the above Resolution that a copy of this Resolution should accompany every scholarship, fellowship, traineeship, and assistantship offer.