That could soon change. WolframAlpha has just released its API which will allow developers to tap into WolframAlpha's computational knowledge base and integrate it with websites, mobile apps and more.
The WolframAlpha API operates as a REST-style webservice and can be used with any programming language that supports HTTP and XML. This means that the most popular web languages like PHP, Ruby, and Python can all interact with WolframAlpha's base system to bring up results or use the computational power to give customized readings.
WolframAlpha's upcoming iPhone app is being built using the API to demonstrate its capabilities. A native app should also be a big improvement on the iPhone optimized version of WolframAlpha that is currently available.