Micro-four-thirds sensors are nowhere near the same size. That said do they overall perform quite well, but not quite as well as an entry level DSLR.

Only mirrorless cameras with APS-C sensors are the Sony Nex (fun to use) and the new Canon EOS-M. The downside of the Sony is the limited range of lenses and the unusual handling. I guess that is a love/hate type of thing - I played around with a NEX7 for some minutes and found it fun but pretty confusing. The EOS-M seems technically pretty much the same as a 650D - which is good.