If you're going to consider buying into a whole new system, and you seem to care a lot about the weight of your new kit, I'd recommend having a closer look at the various mirrorless alternatives (Micro 4/3 from Panasonic/Olympus, Fuji's X-Pro line, or even the big bucks on a Leica M9). It's hard to beat the weight savings. Most of these cameras (with the notable exception of the Fuji and Leica) have D-SLR-like super-fast autofocus systems. Some of them (notably the Fuji and Leica) have UIs that resemble traditional cameras rather than fluffy "kids and pets" modes to gum up the works.

(The Fuji X-Pro 1 doesn't have a built-in flash but has a fancy hybrid viewfinder. The X-E1 nukes the hybrid viewfinder, shrinks the camera, and adds a flash. Had the X-E1 been available from the beginning, I would have purchased it instead of my X-Pro 1.)