enabling core dumps

"Writing R Extensions" says

{quotes}
If you have a crash which gives a core dump you can use something like

     gdb /path/to/R/bin/exec/R core.12345

to examine the core dump. If core dumps are disabled...
{unquotes}

sadly it doesn't go on to say how to enable if core dumps are disabled.

I understand that in bash I need to do

$ ulimit -c unlimited

but this doesn't seem to be enough, I still don't find a core file despite

  *** caught segfault ***
     address 0x20000028, cause 'memory not mapped'

    Possible actions:
     1: abort (with core dump)
     2: normal R exit
     3: exit R without saving workspace
     4: exit R saving workspace
     Selection: 1


I am running Ubuntu jaunty on a laptop. Any ideas as to what I might need to
configure next?

thanks
David