Thank you, Lucia and Ren!
I will definitely take Ren's advice and write an email. SAIC did not provide me with much information; the letter says that they will provide me with more information once they determine if there is space in the class, but there's no mention of when that might occur, or whether their wait list is in numerical order.
A question on formality: who should I address the email to, if sending it to the office of graduate admissions?