Right now, Orca interrupts speech on every key press.  The following thread on the Orca User's mailing list requests that we be able to limit which key(s) are used to interrupt speech:

http://mail.gnome.org/archives/orca-list/2007-October/msg00259.html

This may be as simple as modifying orca.py to only call speech.stop() under specific key event conditions.  I suspect, however, that if we do this, changes will be required all over the place to interrupt speech based upon other AT-SPI events.