The problem disappeared after recompiling libICE with ipv6 support enabled
(and reinstalling qt-core-4.8.2 and smplayer-0.8.0).
(Actually both libSM and libICE were reinstalled, but a look at the libSM
build log shows that ipv6 support is still disabled for libSM, on a system
where ipv6 support is generally not enabled. The libICE build log,
however, has "--enable-ipv6" in the configure options. The previous
version had "--disable-ipv6" instead. smplayer works with qt-4.8.x with
ipv6 support enabled when building libICE-1.0.7 and segfaults without
ipv6 support enabled when building libICE-1.0.7.)
Clearly a qt artifact rather than an smplayer bug (since it worked with qt-4.7.4
either way), but smplayer apparently exercises an obscure qt corner case
related to the presence or absence of ipv6 support in an X library on which qt depends.
Cheers.