You can license this technology for use in your own application*. RhymeBrain Reader is an easy to use C++ library, and a 12 to 24 MB word database, depending on the features you need.
At this time, we do not offer the API as a web service. Any such use would cause our server to turn into a molten pile of mush. However, if you are interested in using Rhymebrain as an API, let us know and maybe we can work something out.
Contact us to pitch your idea.
* Rights to use RhymeBrain technology on the iPhone are not available at this time.