Props to Marcus Oh for this one - too cool to not repost... (and so I can find it more easily when I need it!)

wmic /node:"servername" /user:"domain\username" rdtoggle where servername = "servername" call setallowtsconnections 1
(mind the wrap)

/node: indicates the remote server name
/user: indicates who to grant access to
setallowtsconnections: indicates to enable terminal services. 1 enables it. 0 disables it.