gdb debug of mongrel rails ruby segfault
gdb /usr/local/bin/ruby [PID]
After gdb has attached, be sure to run the "c" command to continue and allow mongrel/ruby to continue running until the segfault, whereupon you can use "whe" to get an extensive backtrace leading to the problem.
Normally coredumps are not shown in a terminal window. To enable core dumps for more information, run:
ulimit -c 512
which allows coredump's upto 512MB in size.