I know what you mean. However, all the +/-/~ signs mean on the gentoo site are that someone has created an ebuild for the package for that particular platform and it works. Same thing here.
If you go to http://packages.debian.org
, you can search for a package (from stable/testing/unstable/any) and it will show you which platforms it has compiled and run on (as a precaution, check the bug reports for packages in unstable). This means that the program DID compile (at the very least, since debian is a mostly binary distro). Most decently-designed apps don't need much change except for a recompile on the target platform (and someone to test the resultant file). For example, I just did a random search for something mozilla-related:
testing (web): A stylesheet editor for Mozilla Composer
0.4.0-1: alpha arm hppa i386 ia64 m68k mips mipsel powerpc s390 sparc
unstable (web): A stylesheet editor for Mozilla Composer
0.4.0-1: alpha arm hppa hurd-i386 i386 ia64 m68k mips mipsel powerpc s390 sparc
As this states, you could run this program on an i386 running Hurd ONLY if you are running debian unstable. However, both version will run on an Itanium (64-bit platform).
Of course, all this is moot, since debian hasn't officially added amd64 support to their unstable branch, much less their more stable branches.