GainSpan designs Wi-Fi modules and the whole software ecosystem. You could prototype things pretty quickly as they already design the whole chip-level firmware and application-level software that goes into building a prototype.
what's really neat about GainSpan chips/systems/software is that they built everything from ground up, so their software is optimized for the embedded space. Sure you could take an ARM-core and design the whole circuit board, and etc, but why would you want to do that?
once you get started, you will then probably realize that to differentiate your prototype is the software and not the hardware. if your wireless device can't perform the correct provisioning mechanism to communicate with the router, you're going to have to spend a lot of time on that too.
well, anyway, I have to disclose that I used to work for a Wi-Fi chip company before we got acquired and I still sometimes do consulting at GainSpan. check out the website and get an evaluation kit to start tinkering. the EVK is fairly inexpensive and it'll help you understand the design process.
best lien