Frank Terbeck wrote a patch for cmus that adds support for Audioscrobbler/Last.fm, then it was taken up by Clay Barnes for a time, with the hopes of importing it into mainline. This never happened and it once again fell by the wayside. I have updated it for my own use, and uploaded it to github in the hopes that it may one day make it to cmus mainline.
Get a current copy of the cmus git repository to patch:
git clone git://gitorious.org/cmus/cmus.git && cd cmus
Patch the cmus tree:
patch < ../cmus-audioscrobbler/cmus-as.patch
The build / install cmus as usual:
./configure && make && su -c 'make install'
It's pretty simple. Just add these lines to your ~/.cmus/rc file:
set as_enable=true
set as_user=(Your Last.fm Username)
set as_pass=(Your Last.fm Password)
Alternatively, just type them as ":set " commands in cmus.
Toggling listening statistics reporting off/on is accomplished by ":set as_enable=false"/"set as_enable=true" respectively.