It would be very handy to have a second SD-card slot, as using the Raspberry without a second SD-card slot there are a lot of things you cannot do, such as backing up data to another SD-Card, downloading and creating a new Linux disto on a second card, resizing a partition on a card, etc etc. Obviously you could connect a card-reader to the USB slot, but if you do that you need a third USB slot, or you need to abandon the mouse, and only use the keyboard.
There IS a simply way to add a second SD-card slot though, although access to it would slower than the normal SD-Card.
On the GPIO is an SPI interface (with two device selects), you could wire that to a second SD-card slot, and that should work, as normally SD-cards can also be driven *though slower) through an SPI interface. Obviously A driver is needed, but there are many more systems that use SPI for the SD-card interface, so a driver should be easily adaptable.
This card slot could be put on a small PCB so that it sits above the original SD-Card, and would be very cheap as al the electronics is already available, the GPIO port should be able to provide enough power, so for the price of the connector, and a very small PCB this could be added to the Raspberry PI.