I was somewhat confused by the fact that gthumb no longer popped up automatically when I plugged my camera in after upgrading from Debian GNU/Linux 4.0 (Etch) to 5.0 (Lenny). Google searches offered no hints so I was forced to dig into it myself.
It appears that under Etch I'd ticked the "Always Perform This Action" box when selecting "Import Photos" from the popup. This caused the /desktop/gnome/volume_manager/prompts/camera_import_photos gconf key to contain the value 5. It seems that this value causes the arrival of the camera to have no visible effect in the Lenny version of gnome-volume-manager.
The fix is to run gconf-editor and reset the aforementioned value to zero so that the popup appears once more. Plugging the camera in again and ticking the box again then results in the value 6 being written back.