Ahem.... as stated numerous times everywhere in the Users list, Dev list, and on this forum:
Roundcube Webmail is a Web 2.0 application that is making the desktop move to the internet.
So, it uses JavaScript to make requests in the background to check for messages, as well as do 99% of the functionality. Yes, it is possible to make a WAP addon, but why? For what point and purpose? The draw of Roundcube is the design, fluidity, and functionality of the system. WAP would destroy the design, fluidity, and functionality all at once. Basically, with WAP you'd get text with links. Even then, subject lines longer than "Re: Uselessness" would wrap onto two or three lines.
Honestly, if you use Roundcube, you're more than likely using a laptop or desktop. If you want WAP functionality, find a WAP mail program. Or use the one in your phone. Developing for WAP would essentially fork the current build system, which is something we can't afford right now.