I haven't released anything a capella in over a decade. Solo multitrack a capella is one of those things you can't perform live, which is one reason why I believe recorded music and live music are separate art forms. However, the idea of performing a polyphonic a capella piece live is still an interesting challenge. Of course, one can use delays or looper hardware, but I've often found the results lacking interesting compositional structure.
The recent Imogen Heap on David Letterman video got me thinking about performing an a capella piece in conjunction with a monome. The goal was to use just the monome and a microphone. No other gear or touching a computer is allowed. Also, no sequencers running backing tracks. This is somewhat of a challenge.
I recorded this as a two camera shoot - a Canon 5DmkII with a lens baby pointed at my head... well... the mic, and another video camera pointed at the monome so you can see what my hands are doing.