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I hope this won't be getting too complex. :)
Firstly, I agree that this is perhaps not a bug at all. I reported it
because I seemed possible that Python2.x had a deliberate workaround for
this issue which somehow got lost in 3.x.
Secondly, I didn't mention that I'm running OpenSolaris on KVM since I
can't get it to install on physical hardware. However, I don't think KVM
plays a role here since gcc produces 'normal' results.
So, the problem combination is:
Ubuntu64bit -> KVM -> OpenSolaris32bit/suncc/Py3k
On the C level, here's an example:
#include <stdio.h>
#include <math.h>
int main(void)
{
 double x = NAN;
 printf("%f %f\n", x, copysign(1.0, x));
 return 0;
}
This gives -NaN, -1.000000 with suncc and NaN, 1.000000 with gcc. 
Back to Python:
sys.float_repr_style is 'legacy'.
It looks like the C99 fenv functions are pretty much supported, but I
don't know since when. I found man pages from 2003 for fenv and from
2006 for the fe*() functions.
This for example works:
#include <stdio.h>
#include <stdio.h>
#include <fenv.h>
#include <assert.h>
int main(void)
{
 #pragma STDC FENV_ACCESS ON
 int save_round;
 int setround_ok;
 save_round = fegetround();
 setround_ok = fesetround(FE_DOWNWARD);
 assert(setround_ok == 0);
 fesetround(save_round);
}
I could test changes that you make (preferably in p3k head, so I could
just pull them). But as I said: If the 'proper' behavior in 2.x was not
a deliberate workaround, there's really no need to change anything.