I've been toying with this, and the problem *is* readline. Readline overrides a number of signal handlers, in order to do cleanup when exiting, and apparently does not restore them on exit :-S The signals readline 'overrides' the handlers of are:

SIGINT
SIGALRM
SIGTSTP
SIGTTOU
SIGTTIN
SIGTERM
SIGWINCH

Which means that the signal handlers for these signals *vanish* when using readline. Interactive mode uses readline, but scripts can themselves, too. I'm not sure why this isn't showing on BSDI, but it might have something to do with the BSD vs the SysV signal handling routines. I'm also not sure if it's possible to fix this problem; it might be a bug in readline, or it might be documented behaviour. But I wasn't able to find a reference to signal handling in the 2.2.1 readline info files.