[Update: BitPenny relaunched and as-of October, 2011 exists as a pool that uses a custom, open-source mining client.]

One month after the second round of testing began, the Bitcoin mining pool BitPenny has shut down, indefinitely.

BitPenny's operator blamed "continued and unsustainable losses" as the reason for discontinuing service.  The service could appear in the future under a different model, according to the announcement that the service would be closing.

The service's method for compensating miners was particularly vulnerable to an attack where a solved block is held back from being submitted and as a result the pool is harmed.

Withdrawals from account balances will continue to be available, and the #bitcoin-bitpenny IRC channel will remain open.

There are now four public mining pool operators and the total network hashing represented by the pools has been increasing steadily.

BitPenny web site

Wiki article