The old webcam was really a big annoyance - unplugging made it most of the time stop working. Then some module reloading and plugging (and luck and patience) was needed to get it running again.
The Logitech Pro 4000 works with the same driver (pwc), doesn't make any problems (yet) and has better image quality! :)
I can reach the other machines on the network by their hostname. The Fritz!Box DHCP/Name-server does consider the hostnames given to the DHCP-clients. Only debian doesn't configure the DHCP-client out of the box to provide the hostname. The two gentoo machines do it without any problems.
So, I'm sorry! ;)
Gentoo explains the debug USE-Flag like that:
Tells configure and the makefiles to build for debugging. Effects vary across packages, but generally it will at least add -g to CFLAGS. Remember to set FEATURES=nostrip too
Well, for KDE-packages that mainly means: Dump everything to .xsesssion-errors. I realized this being the culprit of my overblown .xsession-errors about a month ago - but today this file grew by 4GB in about 3 hours and that surely was too much for me. Reason for that usually is KMail, which is a good mail-client, but surely sucks internally. Having kio_imap4-processess hanging around, doing nothing else but eating 100% of processor-time.
Suggestion to everyone: At least build KDE-packages without debug-flag.
It's just sad, that you can't call the programs with --debug or something like that - it has to be compiled with it. :(