Having the odd HDMI cable around I did not want to splash out on another HDMI cable, so instead opted for a HDMI adapter (Micro Male to Full HDMI female) that cost less than a few pounds.
As you may well move your Surface RT about I would be careful of not snapping or bending the connector, actually a good reason why adapter may well be a good idea, wrecking a cheap adapter not a whole cable is far more bearable though.
The side of the Surface RT is angled so the adapter is not flush against the device (I wonder if a hobby drill could be used to slightly trim plastic away for a better fit?)
Andy