KDE 4.2 warns for (as far as I can tell) every single kde-base package about the use of the kdeprefix use flag. While I appreciate that the developers want to inform users about this, I believe this is excessive. It particular affects users who are using elog functionality (via email in my case), causing a mail to be sent for every single package (most containing only this warning), with no ability to selectively disable this message (while still recieve all the other elog messages). I propose that the kdeprefix useflag warning should only be shown by the kdelibs package. This package is installed by every install that uses kde, so users will still get the message. However it will now only be shown once for each time kde is upgraded instead of for every single package.
Put the following line into /etc/make.conf and the messages will vanish: I_KNOW_WHAT_I_AM_DOING="yes"