Well, actually there's apt-build, a front-end for apt-get to install source packages and wrap gcc to add your own flags. You don't get use flags like with gentoo, but can optimize for architecture/cpu and use Cflags.

I'm playing with this now, using

-mcpu=athlon-xp -march-i686 -O3 -fomit-frame-pointer -ffast-math

Now its got a few kinks in it. Some packages don't have source available in the sources.list I have. Others you might get yourself into trouble if you upgraded - libc6, knoppix-specific stuff, gcc, openoffice.....

Its great for installing single optimized packages. Its also got a world option to update from a list (which you need to create).

Using apt-get upgrade or dist-upgrade would replace your optimized packages, unless you pin them with a higher priority, but then pinned shared libraries might prevent upgrading other packages with apt-get, so that can give you a little trouble.