Hello Joe, no only Leica M mount, there is also the leica C mount, R mount, Contax C........the list goes on.
m4/3 can use a wide range of lenses via adapter. Cost of the adapter is about 600.
Hey Dchiu, i recall you wanted 5D MkII, and i wonder if you want the m4/3 as a substitute or as a P&S.
There is no way that m4/3 can replace a 5DMkII is terms of quality and performance. It is a great and handy camera that performs close to an entry level DSLR while it is 1/2 the size. the bonus is that you can get adapters to use old manual focusing lenses (recently, also Contax G lenses).