Directory pull is available on new android tools. ( I don't know from which version it was added, but its working on latest ADT 21.1 )
adb pull /sdcard/Robotium-Screenshots
pull: building file list...
pull: /sdcard/Robotium-Screenshots/090313-110415.jpg -> ./090313-110415.jpg
pull: /sdcard/Robotium-Screenshots/090313-110412.jpg -> ./090313-110412.jpg
pull: /sdcard/Robotium-Screenshots/090313-110408.jpg -> ./090313-110408.jpg
pull: /sdcard/Robotium-Screenshots/090313-110406.jpg -> ./090313-110406.jpg
pull: /sdcard/Robotium-Screenshots/090313-110404.jpg -> ./090313-110404.jpg
5 files pulled. 0 files skipped.
61 KB/s (338736 bytes in 5.409s)