Embedded devices are ubiquitous in our environment, including computing systems as widely diverse as digital watches, automobile dashboards, factory controllers, thermostats and other appliances. Traditionally, little attention has been paid to the user interface in such low-cost, dedicated-function devices. However, new technologies such as touchscreens are changing the landscape of embedded user interface design. Additionally, recent research has demonstrated that usability can have a significant effect on embedded device efficiency. Research on programmable thermostats in particular points to the need for proficient and consistent user interface design in order to realize energy savings nationwide. We discuss preliminary results from an in-depth usability study conducted on five programmable thermostat interfaces (three touchscreen, one web, and one-button based) with 31 participants. Our research suggests that users lacked a consistent mental model of how to interact with buttons, text, icons, and other features of the device. We hypothesize that discrepancies between perceived and actual affordances on the device had a measurable impact on users' ability to successfully accomplish key tasks on the thermostats.