A set of guidelines that distill current practice in building speech interfaces at Carnegie Mellon is available (also as a PDF file). These guidelines have been found effective for interfaces that are part of desktop computers, this being what we are the most familiar with. They may be less appropriate in other contexts, such as telephone-based interfaces; on the other hand, they catalogue a variety of common-sense features which should be useful across various implementations.
This project explores speech-only interfaces for devices for which traditional interface elements (keyboards, displays) are lacking but which nevertheless are meant to be used for complex tasks. Wearable computers are an important class of such devices.
Since such devices lack GUIs, the interfaces begin to look less like traditional interfaces and begin to assume some of the characteristics of human interlocutors in their ability to maintain a communication channel with a human user. A short working paper on this topic is available.
More information may be obtained from Alexander Rudnicky.