digikam is based on libgphoto2, AFAIK. You need write access to corresponding USB-port. This can be achieved via hotplug (look at the gphoto2 documentation) or allowing USB write on your usb filesystem. In order to achieve the last one (only 2.6 kernels) you have to have a line in your /etc/fstab as
usbfs /proc/bus/usb usbfs defaults,devmode=0666 0 0
Note devmode=0666 gives read-write access to everyone for every usb device. However, I am unsure if it works for kernel < 2.6.8 (it didn't work for 2.6.5 due to a bug).
With hotplug, much more is possible.