I spent a week recently working on the nonfree firmware problem in debian-installer. The problem of course, is that nasty nonfree firmware is needed to use many wireless cards, some other NICs, and some disk controllers, but cannot be included in d-i images.
The fix/workaround landed in unstable today. d-i will detect if a driver needs firmware that is not available. It'll then prompt the user to put the firmware on removable media (such as a USB stick, or floppy, or really anything you can plug into a machine without drawing blood).
The debian-cd team will be building some zip files and tarballs containing all the firmware that debian ships in non-free. (images here) Extract one of those onto a USB stick (vfat formatted), and you're good to go.
The installer will also tell you the names of the firmware files it needs, so if they're some of the ones that Debian cannot even legally distribute (such as the ipw2200 firmware), you can go hunt them down, and dump them onto your USB stick too.
The installer can find firmware files loose on the USB stick, or packaged up in a .deb (or .udeb). The firmware will be used during the installation, and also installed into the finished system. So providing it in .debs is cleanest.
(BTW, if you package a firmware deb for debian, please avoid putting anything but firmware and documentation in it. These firmware debs are also installed into the d-i environment now.)
This could be considered a little clumsy, needing to manually find the bits and put them on a USB stick. I think that people who install Debian frequently will probably end up gravitating toward the "hd-media" installer that boots from USB stick. The firmware can then be put on the same USB stick that contains the installer, and only one thing be needed to install.
Still to do: Use discover's capabilities to scan for firmware that is needed by other devices on the machine, that are not needed to install, but can be used later, and install that firmware too. (This will also support installing non-free kernel modules and other such evil.)