It seems most of us thought this was some proprietary port that required their official adapter, but looking at the leaked pcmag review;
"Microsoft calls it a "Micro HD" port and sells a Micro HD-to-HDMI port adapter ($39.99 list), as well as a Micro HD-to-VGA adapter ($39.99 list). However, when we hooked up an off the shelf micro-HDMI-to-HDMI cable to the Surface, we were able to hook up to and use a HDMI monitor without any issues. The Surface supports extending Windows RT to the external screen, as well as mirroring the tablet's screen on the external display."
Designed for Surface with Windows RT, this proprietary HD Digital AV Adapter lets you connect your device to any HDMI-compatible display (HDTV, monitor, or projector) and stream movies, play video games, and watch slideshows in high definition.
Pretty sure RT won't do multiple monitor setup while the Pro will
It does. In the pulled PCMag review, they also stated you can do extended or cloned displays on the RT:
The Surface supports extending Windows RT to the external screen, as well as mirroring the tablet's screen on the external display. This means you can use the Surface in a dual-monitor configuration, just like a desktop or laptop.