To be successful, each mode of a multimodal application must add value, be supported on widely available devices, and be supported by a widely available browser. Each new mode must add value to the application in which it is used. Leverage the strength of each new mode. There are many promising multimodal devices waiting to be implemented on integrated cell phone/PDA devices and PCs.
For a further discussion of multimodal user interfaces, see the last chapter in the author's book, VoiceXML: An Introduction to Building Voice Applications (Prentice Hall, 2002, ISBN: 0130092622).