Canon and Nikon dwarf Olympus and Panasonic, so they have some time to react, but they clearly need to. The problem is that a new mirrorless system requires a new line of lenses to provide the true benefits of the downscaled format, but both companies already have two existing lines of lenses--one for their pro-level, full-frame sensored cameras and one for the consumer DSLRs cameras with APS-sized sensors. Having a third line of lenses may be too much, so there's a fair chance we'll see the Big Two go in a different direction.
Instead of focusing on another interchangeable lens format, the companies may release truly compact cameras with built-in zoom lenses much like those in their existing PowerShot and Coolpix lines, but with large consumer-level DSLR sensors. This makes sense, because ultimately, this is what consumers want--as they showed in the film days. Most digital camera sales still tend towards compact units; as nice as the Micro Four Thirds cameras are, they don't slip into your pocket. You need to make a conscious decision to carry them around.
Two small camera manufacturers--Sigma, primarily known for its after-market lenses, and Leica, known for its very pricey premium cameras--were first out of the gate with all-in-one big-sensor cameras, the Sigma DP1 and DP2 and Leica X1. All unfortunately use fixed-focal-length (non-zoom) lenses, which limit their appeal (as will the Leica's $2,000 price tag). Until these cameras can incorporate zooms, they'll be limited to a very small enthusiast market.
But once they do (and my guess is that this will happen in mid-2010), watch out. Consumer-level DSLRs won't go away; they didn't in the film days, and they won't now. But they'll become marginalized as more and more people turn toward more convenient alternatives. History has a way of repeating itself.