sources.list for debian sid source code

[Cameron Hutchison]

I'm trying to figure out how to set up my /etc/apt/sources.list so that
I can add a deb-src line for Debian unstable. But I only want to fetch
sources from that source when explicitly specified.

So if I do an
$ apt-get source foo
it should pull the sources for foo from dapper sources (as I currently
habe it set up).

I'd like to do
$ apt-get source xterm/sid
or something like that to get the sources for xterm from Debian sid.

I want to be able to selectively backport sources from debian unstable.
At the moment I only need to do this with xterm (xterm-208 in dapper has
a bug that affects me that was fixed in xterm-210) - and I understand
the potential problems that may arise given that Ubuntu is not Debian.

I think the way to do this has something to do with apt preferences and
pinning, but I'm not sure how to set it up.

Thanks