I liked a lot the feature, Ubuntu detecting automagically my camera so I thought about implementing it in my new Gentoo system too. But I had no idea how to do it. It turned out to be very easy in fact.

At first you need gnome installed in your computer. I will not talk about this part here.

You also need the gnome-volume-manager installed. in gentoo you need to type:

emerge gentoo-volume-manager
rc-update add hald default
rc-update add dbus default

Then start the hald, and dbus services.

I like to use gthumb to download my pictures from my camera (Just like default Ubuntu) so I installed gthumb:

emerge gthumb

To setup which program to be run when a camera is plugged in, run:

gnome-volume-properties

and from the Cameras tab, checkmark "Import digital photographs when connected" and enter "gthumb --import-photos" to the Command: field. Click Close.

Update: this step is especially important for PTP compliant cameras, if your camera is recognized as a usb drive, gnome-volume-manager will possibly automount your camera as a drive anyway. 

We have one more step to go. (And I spent some time to find it out...) add your user to the group declared in: /etc/hotplug/usb/usbcam (In my case it was plugdev)

Next time you insert your camera, TADAAAA! you will automagically have the Import Photo dialog appearing:

Good luck!