My wife looked into this recently specifically she wanted to be able to copy/move stuff from her camera to an external USB drive. Good info on
http://connect.dpreview.com/post/8241012583/tablets-for-photographers . The main concern was for a full-size USB port for external devices.
She rejected the iPad quite quickly for extremely limited features and as overpriced. The Toshiba Thrive looked the best bet with a full size USB, SD card reader and other stuff, but then a Groupon deal popped up on an inexpensive Acer with two USB ports (IIRC), so she rushed into that (only to find it had a microSD reader).
The simple solution (and one we used in KZN, where she took a few hundred GB of photos) was a USB hub with a card reader and a 2.5" external drive. Sidesteps the issues of limited storage and the fact that tablets are more than a little iffy at reading directly from cameras. Of course, with a decent connection, cloud storage removes the need for the USB drive, but you can't always rely on a good connection when travelling.
Also - most of the docking stations remove the need for a USB keyboard and have USB ports/hubs built in - a definite option.