I currently have a few projects which are bound to servers so I can control them with web apps and this seems like a great way to liberate them from the network. Low-level analog signal processing is about where my knowledge comes to a screeching halt (hence using Arduinos in the first place) but I'd love to play around with any AVR code to try and implement this for high-level language hardware people.
Since you are already taking the flash hit with sprintf, you might be interested to know that you can redirect stdout to print all data through your serial line, i.e. just using printf. Saves going through an intermediate buffer. Looks something like this:
https://github.com/NeoCat/FSK-Serial-Generator-in-JavaScript...
Also, s/buad/baud/
Edit: Awesome project. (: