The native apps that support it seem to have a app-internal selector - I'm guessing that it'll be like LED flashlight apps. Available on an app-by-app and phone-by-phone basis until some future release where the API is more standard and then more apps will be able to support all phones.
Unfortunately there's some hysteria over HDMI among content owners, since it's digital out they don't like the idea of just anyone enabling an HDMI interface (silly, I know, but blame the content owners and not Verizon/Motorola). Given this, I'm not holding my breath but we can always hope.