OK, I checked the contents of collection.db with sqlite3 and sqlite3_analyzer.
The table exists but is completely empty. That's really odd because even after deleting ~/.kde/share/apps/amarok/ and amarokrc this table never contains any data.
I have uploaded my db at http://buick6.de/stuff/collection.db.tar.gz.

to figure this out i need amarok's debug output. the kubuntu package was probably not built with debug output enabled, so you'll have to install it from source. There are instructions on the homepage. Please start amarok, scan your collection, and close it again and post the console output (or upload it somewhere). I'll be away till next week so I won't be able respond, sorry.

OK, Amarok shows TONS of Debug output on the console. If that is not what you want I will compile it and post it here.
http://buick6.de/stuff/amarok.txt

So far, thanks a lot for your patience. I guess we will figure it out one day

Sorry for the late reply. I was away for almost a week. I checked the log you posted and amarok cannot load the plugins needed by Dynamic Collection. Make sure that run kbuildsycoca after running make install.

Just to close this thread:
Running kbuildsycoca didn't help but using the official kubuntu amarok packages (released on 06.09.) helped.
Now everything works as expected. Completely transparently. Amarok continues to amaze me gain and again