With the kdebase-kioslaves package, KDE gets some automounting support. More importantly, you can mount / unmount media straight from Dolphin.

HAL

As of June 2010, the current way to get automounting on KDE relies on HAL. Everything should work automatically as long as you have the dbus and hal USE flags, hald service is started and your user is in the plugdev group. You may need to install the pmount package (probably not, though).
Note that as HAL is deprecated, functionnality should move to udev and I hope KDE support will be based on that.

Old information

Follow this Gentoo HOW-TO.
Make sure your normal user is in the groups plugdev, haldaemon and messagebus. The latter two are not mandatory according to the HOW-TO but I think it's best to have them anyway.
Note that having mount folders for removable media in /mnt is deprecated. The FHS standard recommends using /media/.

When navigating to the media:/ URL in Konqueror, Konqueror displays a list of all the devices found in /etc/fstab, if they are present in the system (plugged in). It is important to understand that the only configuration file involved is /etc/fstab (KDE calls a program called fstab-sync to know when the file has been modified).
Make sure your fstab line is correct (notably that it contains the user option). Also, when providing the device location, use the real location, not a symlink. Apparently a symlink can cause troubles.
Once this is done, when navigating to media:/ you should see a list of all your media. Double clicking on any of them should mount them.
UPDATE (15/11/2007): With newer versions of KDE or hal, you don't need to even have a line in /etc/fstab. I also don't think the fstab-sync trick is used anymore, KDE should get the information via HAL.
If it doesn't work, try the following (I don't know exactly which one solved the problem):
- Install pmount. Reboot.
- Remove the CD-ROM line entirely in /etc/fstab.
- Build your kernel without ATA support at all (use PATA support for CD-ROM).
- Rebuild dbus-qt3-old, then kdebase-kioslaves. This is probably not the solution.

Don't put empty lines in your /etc/fstab. Apparently, if your media lines are after an empty line, KDE won't recognize them.

What is unclear to me is how the automounting system of KDE works. Sometimes a popup will appear asking for a default action. I don't understand exactly how this works and how can it be configured.