Along with the new campaign, new multiplayer units, and a Swarm Host of other additions, StarCraft II's Heart of the Swarm expansion will include some tools to create custom interfaces for use in observer mode. Elements of the UI that are designed to help you play the game, but don't necessarily add the experience of watching it, can be shrunk, moved, or axed completely. Fonts can be swapped, and custom assets can be inserted.
This will be particularly significant for streamers, casters, and tournament organizers. Instead of relying on self-made overlays to corral things, casters will be able to build a UI from the ground up. At best, this could improve the viewer experience and grant specific channels, casters, and events a unique feel, and maybe even help with monetizing through sponsor placement.
These options won't be available from within the game client itself. You'll have to get a bit fiddly with some of SC2's resource directories. If you're interested in what will and won't be possible, this post on the Blizzard forums explains in a little more detail. Just be prepared, as it's geared more toward experienced modders, and doesn't do a great job of demonstrating how the functionality translates into the game.
Heart of the Swarm remains burrowed before its March 12 release. I'm planning to cloak inside Blizzard's main—I'll be visiting them in Irvine, CA tomorrow—and collect more intel, which I'll post up as soon as I can.