Okay, I have remote desktop enabled in the Windows guest, and using
rdesktop in the Fedora host, I can copy and paste between them. This
has the additional benefit of being able to have the guest desktop
matching the size of the host desktop; I can do:
rdesktop -g1920x1200 ip_address_of_guest

However, this just works for Windows. How do people solve this
problem for all guest operating systems? What is the best practice?

Hi:

Here is my script for starting my VM:
export SDL_VIDEO_X11_DGAMOUSE=0
export QEMU_AUDIO_DRV=alsa
kvm -boot c -hda /home/jserink/winxp1.img -cdrom /home/jserink/CDs/winxp.iso -m 1512 -net nic,vlan=0,model=virtio -net user -localtime -soundhw es1370 -no-quit -vga cirrus &
/home/jserink/qgt-2005-03-02-19/host-linux 127.0.0.1 > /dev/null 2>&1 &

I use host-linux to enable copy and past (text only) between my VM running XP and linux. Works great.
host-linux comes from the Qemu-Guest-Tools, QGT. its very dated, from 1995 but works great. You get the windows executable and the linux source, build it and away you go.
Its available here:
http://wolfpackally.wo.funpic.de/qemu/qgt/

It looks abandoned but it works fine.

Cheers,
john