Does the gumstix buildroot build use gnu libstdc++, or is there some 
uCLibc equivalent that doesn't care.
Having just done the buildroot thing, I can tell you that the libstdc++ is coming from gcc (not uCLibc).  It looks like --enable-threading is only set if you select linuxthreads, not NPTL, during make menuconfig.

Check me on this: NPTL sets BR2_PTHREADS_NATIVE=y in .config, which as far as I can tell doesn't trigger anything special, whereas linuxthreads sets BR2_PTHREADS=y in .config, which then triggers THREADS=--enable-threads in toolchain/Makefile.in, which is then used in toolchain/gcc/gcc-uclibc-3.x.mk when calling the main gcc configure.  So that *should* be what we need for thread support, yes?

So, why doesn't NPTL trigger --enable-threads?  Is this an oversight?  Perhaps this is a better check from toolchain/Makefile.in:
ifeq ($(BR2_PTHREADS_NONE),y)
THREADS:=--disable-threads
else
THREADS:=--enable-threads
endif
(in other words, disable threads if no threading model is selected, otherwise *something* is selected, so enable support...?)

Thanks for pointing this out, our code is C++ and uses threads as well, so I'm about to run into this.

-ethan