Unfortunately, unless it is deemed a security fix, this change will never hit Debian Lenny. The stable version is specifically not updated except to patch security holes. You would have to build it from testing to get it on Lenny (add a deb-src for squeeze in sources.list, sudo apt-get build-dep bastille; sudo apt-get source -b -t testing bastille).
Ah yes, thanks. I should have known that - that's the reason why I picked Debian over Ubuntu in the first place; they're more conservative about what goes into stable.
I'm a bit hesitant about trying software from testing, but I might make an exception for Bastille.