So I posted a little summary in English of the buildings and trees tracing and importing in Szczecin on my blog: http://unadventure.wordpress.com/2010/06/20/mostly-done-with-szczecin/

Posted it there because it's more from a personal point of view than a technical summary. You can find another summary, in Polish, but including some nice screenshots at the OSM blog at blog.openstreetmap.pl/2010/szczecin-najbardziej-kompletnym-miastem/

Now I need to catch up with all the things I noted down while surveying in my own city, and had no time to input into JOSM till now.