[Guido]
... Note that C doesn't guarantee that a++ is atomic either, even if a is declared volatile.
For people who don't know, or may have forgotten, threads are not a part of std C or C++: those languages say nothing whatsoever about behavior in the presence of threads. And this is still true in C99! ANSI/ISO committees work by consensus, and there are so many goofy thread models out there consensus will never be reached (keep that in mind as you ponder how PEPs should proceed <0.6 wink>).
(I believe there's an atomic data type, but I don't think it guarantees atomic ++.
You may be thinking of sig_atomic_t? That relates to a technical meaning for "atomic" in signal handlers, a meaning that has nothing to do with threads. In effect, sig_atomic_t is simply a volatile integral type small enough so that read and write each take one machine instruction (so, e.g., a 64-bit int would not serve as a suitable sig_atomic_t on a 32-bit machine, as it would have to be read and written "in pieces", and an interrupting signal could cause an interrupted read or write to use inconsistent pieces).