I haven't heard back and I interviewed the week of Nov. 15th. The ADCOM met on Monday (Dec. 13) for the last time before the New Year. Did UMD tell you that you'd hear before Xmas?
Don't worry if you don't hear back as quickly as others who interviewed the same time as you. It all depends on when your interviewers submit your evaluations.
Oh, and I think it took 4 weeks for me to receive my acceptance letter. I actually called and asked about my status over the phone.