What is Fedora?

Debian, as usual, has this concept -- search for "pinning" -- but it's not as useful as it might sound, because of the dependency chains people mention. (OTOH, it works very well for pulling a few packages from unstable onto a mostly-testing box; you might pull down more than you planned, but it's all stuff that will eventually roll over into testing anyway, so you're unlikely to really destabilize your box, and eventually you'll end up back on pure testing.)

It's also quite easy to avoid those dependency chains in most cases by building an unstable source package by hand. (apt-get build-dep foo; apt-get source foo; dch --local ~blahblah.1; dpkg-buildpackage -b -uc; dpkg -i foo.deb)

Not as easy as gentoo, though :-)