'Sherlock' Mini Episode to Debut on Christmas Day!

My only confusion amidst my excitement that we're getting a Sherlock mini episode (note: my confusion is not what is a mini episode because frankly I don't care since a regular Sherlock ep is the size of a full-length feature) on Christmas is what the heck is BBC's Red Button service and do we Americans have access to it. Anyone know what's the haps with that?