The Model B uses a SMSC LAN9512 USB 2.0 Hub and 10/100 Ethernet Controller IC to provide multiple physical USB host ports from the single USB OTG port available on the Broadcom SoC.
The presence of this hub IC prevents the USB ports from being used in slave mode. However, as the Model A does not have this IC present and instead just has a single USB port connected directly to the SoC's OTG port it should be possible to configure it to act in slave mode through software.
As the Model A has a female host USB port, a HOST to HOST conversion cable will be needed.
Please see this Raspberry Pi Forum thread for more details: http://www.raspberrypi.org/phpBB3/viewtopic.php?f=63&t=15696