Just to let you know that transmission emerges perfectly on gentoo.
Using setup isntructions from: http://dev.gentoo.org/~armin76/arm/sheevaplug/install.xml
All you have to do is:
echo "net-p2p/transmission **" >> /etc/portage/package.keywords
emerge -av transmission
Dont forget to USE = "-gtk -qt" if you only want the daemon.
Settings file is in /etc/conf.d/transmission-daemon
A bit of an update: editing settings.json is no use, the init script destroys it, so you have to edit the above mentioned file.
A quick tip: to disable the rpc whitelist simply open the config file and edit the allowed line to: