I also was just accepted into UMD School of Information Ph.D program, with funding.
I am not surprised at how quickly they sent out acceptance letters; six days after submitting my application in November, I received a request for a phone interview. At that time they said their goal was to get the letters out before the holidays but that it may take them until early January.
The March 1st deadline for acceptance does seem like a nasty trick to get us to accept before hearing from the rest of the schools we have applied to. My plan is to call the Director of Student Services (who signed the acceptance letter) in mid-February if I would like an extension. If they won't, I will be in a bind: do I accept with the potential of backing out, denying funding to another student, or do I decline? But that is an issue for later.
Let me know if you get any resolution from emailing them!