and i quote, "after may 15, schools may require applicants to respond to acceptance offers in less than two weeks. an applicant may be required to file a statement of intent, a deposit, or both. the statement of intent should provide freedom to withdraw if the applicant is later accpeted by a school that he or she prefers. under no circumstance should an applicant be asked to withdraw from waiting lists as a condition of accepting a place"
i have received a few acceptance offers. some have just required me to reply in writing that i'm accepting the offer. others required in writing and deposit. one had a statement of intent form which i signed stating i accept their offer. either way works. just submit what the school asks for. none should ask for anything in writing saying you're not going to attend another school if later accepted. when you respond in writing (no pre-printed form) i believe that's a statment of intent. merely an acceptance in writing. not to be confused with the "i won't accept any other school statement of intent letters" that some are writing to gain acceptance at their top ranked school. that might have some legal ramifications if you later try to take another school.
if you replied as requested by the school after the initial acceptance offer, you're good to go.