Looking at other setups it seems everyone is running with multiple USB Asic Miners through powered USB Hubs. Can the Raspberry Pi power a single miner through USB? Even using a 2 amp power adapter?
I have been running my Pi (B, 512mb) with a 1.5 amp power supply with 1 usb erupter (Sapphire) for over a week with no problems. Directly connected to the Raspberry, no hub.
Put it into a powered USB hub, then connect the hub to the PI.
Looking here it seems that the USB ports on the RPi cannot exceed 100mA so it probably wouldn't be a good idea to use a Block Erupter directly on the RPi
This answer depends on the board revision of the Raspberry for older boards you have a 100mA limit
this limit is no longer valid for Raspberry Model B Revision 2.0 but better use a powered USB HUB without on/off button
Rev 1.0 = MAX 1A
Rev 2.0 = MAX 2.5A