The new Raspberry Pi B+ has an increased power spec for the USB ports, with a special setting as described in the link below up to 1,2A:
http://www.raspberrypi.org/forums/viewtopic.php?f=63&t=81736&start=50#p577877You nede to have a 2A power supply. This then is enough to run an Antminer U2 directly in the Raspberry Pi B+ USB port. I tried it and it works fine! Antminer U2 runs overclocked at 2GH/s on MinePeon OS with following configuration setting:
#!/bin/bash
sleep 10
/usr/bin/screen -dmS miner /opt/minepeon/bin/bfgminer -S antminer:all -o stratum.mining.eligius.st:3334 -u <address> -p none --set-device antminer:clock=x0981