This release has 3450 lines of Arc and 1157 lines of Scheme, so it looks like it's mostly On Arc already. About a C implementation -- looking at benchmarks, I'd be happy if it was just ported to Ikarus Scheme. (Maybe once Ikarus is done...)
Actually, quite a few features Arc uses are taken from the underlying scheme -- like garbage collection, numbers, closures, basic I/O, strings, and so on. In that case, you'll need to count most of PLT Scheme, which dwarfs Arc's 3450 lines. So Arc isn't nearly self hosting yet. But it will be.
