Excellent post Eliezer. I have just a small quibble: it should be made clear that decoherance and the many worlds interpretations are logically distinct. Many physicists, especially condensed matter physicist working on quantum computation/information, use models of microscopic decoherance on a daily basis while remaining agnostic about collapse. These models of decoherance (used for so-called "partial measurement") are directly experimentally testable.

Maybe a better term for what you are talking about is macroscopic decoherance. As of right now, no one has ever created serious macroscopic superpositions. Macroscopic decoherance, and hence the many worlds interpretation, rely on extrapolating microscopic observed phenomena.

If there's one lesson we can take from the history of physics, it's that everytime new experimental "regimes" are probed (e.g. large velocities, small sizes, large mass densities, large energies), phenomena are observed which lead to new theories (special relativity, quantum mechanics, general relativity, and the standard model, respectively). This is part of the reason I find it likely that the peculiar implications of uncollapsed hermitian evolution are simply the artifacts of using quantum mechanics outside its regime of applicability.

Here at UC Santa Barbara, Dirk Bouwmeester is trying to probe this macroscopic regime by superposing a cantilever that is ~50 microns across--big enough to see with an optical microscope!