I am just working through my sieve install, this looks like a good guide, however it may be worth adding support for managesieve, which allows the use of per-user sieve filters controlled by a squirrelmail plugin or even thrunderbird directly.

This needs to be done as is listed here:

http://wiki.dovecot.org/ManageSieve

by patching the dovecot source first....

will let you know my mileage