Tarball binary for download.
Having it on a USB drive would have been the simplest, but I'm partial to installing software on the Chumby's microSD card, like I did for my http server.
To install, ssh into the Chumby:
cd /mnt/storage wget kwf.dyndns.org:5821/files/tftpd.tar.gz tar xfvz tftpd.tar.gz
Configure it to start on boot by adding the following line to /psp/rfs1/userhook1 and reboot:
/mnt/storage/tftp/sbin/in.tftpd -l -s /mnt/storage/tftp/var
Add any files you want available via TFTP on your network to /mnt/storage/tftp/var.
I compiled this from source for tftp-hpa 0.44.