Guess I am getting closer to running a well-usable version of Leopard, but I am not quite there yet: Since yesterday, my Trash turns out to be always shown as "full":
(Yes, Papierkorb is the German word for "paper bin").
So far, I haven't been able to figure out why this happens -- there's nothing in there, really. I even "sudo rm -rf"-deleted the .Trash folder out of my home folder, but it didn't help. OSX did automatically recreate it (empty!) but the dock icon won't change.
For now, I guess I'll have to live with an "overflowing" trash can -- if you have any hints though what else I could do, a comment would be very much appreciated :)
Update: It's fixed! It was a filesystem corruption in the fat32 filesystem. I ran a chkdsk which converted the "undeletable" file into one that I was able to remove. Thanks for pointing me into the right direction, Jean Pierre and Greg!