So i've checked and 0.7.61 seems to have similar behaviour(http://xgame.pl/nginx/)
In logs the only suspicious thing was:
2009/06/25 23:38:09 19083#0: worker process 19088 exited on signal 11
and some others similar, always signal 11.
So how to make coredump? I've got --with-debug package :)
Posted at Nginx Forum: http://forum.nginx.org/read.php?2,3374,3396#msg-3396