[everlastingfan]

Is Arduino firmware open? Are the PCB and chip designs open?

[kragen]

Yes, yes, and no. Also the compilation toolchain depends on GCC. Open silicon designs aren't enough; you need to be able to verify that the design correctly describes the physical hardware you are actually using.

[secraetomani]

Even if not, way less stuff hiding in there than in your average computer. Remember that these days, Intel CPUs have web-servers embedded in them.