Cool! As wikipedia notes, it's harder to design things this way (and apparently tends to use more transistors). But thank you, I hadn't a name to search for before. :)

http://en.wikipedia.org/wiki/Asynchronous_circuit

Harder is not necessarily true. It depends on what you're trying to build.

In terms of QDI Async circuits, the main disadvantage is an increase in wires. In synchronous circuits, a slow enough clock ensures that the computation is completed (propagates through transistors) before moving on to the next clock cycle. For QDI async, the replacement is to have a handshaking protocol to ensure correctness as values pass through the transistors/computation. The handshaking increases the number of wires needed (and thus silicon area).