I have written a managesieve server from scratch in Python. I did just enough work to get it working with avelsieve (the squirrelmail plugin), smartsieve, and KDE's sieve kioplugin. It supports the entire Internet draft. Since I didn't have a working sieve server to compare against, I'm not sure if it's 100% compatible with timsieved. Client applications seem to work okay anyway.
I wrote it in such a way as to decouple how authentication is carried out and how sieve scripts are actually stored, so you could for instance modify this to work with SASL and Exim. Currently it only has code for PAM and Dovecot's LDA, because I run Postfix :). TLS support is in the code but disabled because I had problems writing a unit test for it. All my connections are from localhost anyway.
http://woozle.org/~neale/repos/pysieved if anyone is interested.
I haven't looked closely at Stephan Bosch's managesieve server in C but I do want to acknowledge that he has written a C managesieve server that currently exists as a patch to the Dovecot source code. People looking for a managesieve implementation in C are encouraged to try his patch out: http://sinas.rename-it.nl/~sirius/
Neale