I doubt anyone went through the trouble of making an C decompiler. You might have allot more luck finding and disassembler (that takes the HEX files and turns it into an ASM file). Writing an disassembler for a Harvard architecture would be very easy because there's no confusion betwen data and code but then again it's a bit of work.
Try googling for "AVR disassembler" and see where it gets you.