Just for what it's worth, there is an alternative: Sony's NEX series EVIL cameras are pretty much the same size as micro-4/3rds, but have an APS-C sized sensor. They've recently signed up Zeiss, Cosina, Sigma and Tamron to build lenses and generally support the mount they're using, so while it may not have quite as many companies involved, it's still a fair number of solid companies that should provide a pretty reasonable range of lenses. At a guess, there's a pretty fair chance that at least one of those will start to build bodies as well. Like micro-4/3rds, the flange distance is short enough to allow adapters for almost any existing interchangeable lens.
Edit: I should add that IMO, the reduced size and weight of micro-4/3rds is probably less a result of the format change that of eliminating the entire SLR mechanism. A pentaprism is a big, relatively heavy chunk of glass. The mirror and ability to flip it out of the way dictates a minimum distance from the lens to the shutter, and so on.
I think, however, there's one more factor to keep in mind: micro-4/3rds (and non-micro 4/3rds before it) were/are originated primarily by Olympus. Olympus has been placing an emphasis on their cameras being smaller and lighter than the competition for decades. Even back when they were all building full-frame 35mm SLRs, the Olympus OM-1 (for only one example) was substantially smaller and lighter than the Canon F1 or Nikon F. Likewise, for decades Olympus lenses were generally the smallest and lightest in their category, but were still excellent both optically and mechanically.
In APS-C SLRs, Pentax's line is rather similar. They're substantially smaller and lighter than almost anything else, with no compromise in either optical or mechanical quality. That's not to say that a Pentax SLR competes with micro-4/3rds or NEX in size or weight, but it does show that size and weight can often be reduced without changing formats or losing quality.
Edit2: I have to disagree with the claim that NEX 5 lenses will "necessarily" be any larger or heavier than equivalent micro-4/3rds lenses. At least based on current lineups, it looks to me like most weight differences depend more on features and construction than the difference in sensor size.
For example, it's absolutely true that the Olympus 14-140 mm lens is considerably lighter than the Sony 18-200. It's also true that the Sony uses a metal body, includes image stabilization, and every review I've seen says the build quality is extremely good. By contrast, the Olympus uses a plastic body, lacks image stabilization, and about the best anybody seems to be able to say about its build quality is that it doesn't seem to cause any major problem in real use.
If you want image stabilization and better construction you can get that in the Panasonic/Leica 14-150mm lens -- but it also weighs about the same as the Sony (a tiny bit more, AAMOF).