Which versions of libraries are developers supposed to compile against?

[Jules Bean <jules@jellybean.co.uk>]

So we're heading up to a freeze, and a release, shortly.

At the moment, when I build a package, I do this:

apt-get install `grep Build-Depends ~jules/progging/debian/packaging/balsa/balsa-1.1.7/debian/control | awk -F: '{print $2}' | tr ',' ' '`

...which gets the me the latest (sid) versions of all balsa's
build-dependencies, so that I'm always building against the cutting
edge. I do this on the assumption that packages in sid are destined
for woody.

But when things start being properly, but gradually frozen, presumably 
that simple strategy won't be sufficient.  Because if balsa
Build-Depends on libfoo-dev, and libfoo is frozen, I want the woody
version of libfoo-dev, not the unstable (sid) one.

Is it ever going to be possible to upload straight into woody? For
critical bug-fixes on packages which have already split into two
versions, a frozen one and an unstable one?

Is there some apt 'pin' magic which will make it easy for developers
to do the Right Thing?  If not, I can picture quite a few packages
being uploaded compiled against the wrong things...

Jules