The advent of advanced mobile devices in combination with new interaction modalities and methods for the tracking of contextual information, opens new possibilities in the field of context-aware user interface adaptation. One particular research direction is the automatic context-aware adaptation of input modalities in multimodal mobile interfaces. We present existing adaptive multimodal mobile input solutions and position them within closely related research fields. Based on a detailed analysis of the state of the art, we propose eight design guidelines for adaptive multimodal mobile input solutions. The use of these guidelines is further illustrated through the design and development of an adaptive multimodal calendar application.