What can xxxCoin do to stop the impending 51% attack? Within the coming weeks both Namecoin and SolidCoin have been threatened with the same attack. What can be done by the users, pool operators and developers of these forks to either;

A. Minimise damage.

B. Stop it altogether.
There are two basic fronts on which they can defend, and they really do have to do both.

First, they need to increase their total hashing power. 51% of a very large number is much harder for an attacker to muster than 51% of a small number. Merged mining is probably the most realistic way for these currencies to accomplish this.

Second, they need to fix specific vulnerabilities in their difficulty adjusting algorithms. Some of these attacks requires less than 51% of the hashing power or do more damage than they should because of specific weaknesses in the difficulty-adjusting algorithm.

Alternative currencies are vulnerable to hash rate decrease attacks. To resist them, they've adjusted their difficulty algorithms so that difficulty can drop rapidly. Unfortunately, these algorithms have been found to have unexpected defects. Because strict timestamp accuracy can't be enforced, an attacker can maliciously inject false timestamps into mined blocks to cause the difficulty to drop very, very quickly. This allows a malicious group of coordinated miners to extract a very large number of coins in a very small period of time with relatively little hashing power.

Effort will be needed to fix these algorithms so that they still provide protection against a hash rate decrease attack but don't create even more serious new vulnerabilities.

It may well be a rocky road these next few weeks. The 51% attack is fundamental to the decentralized nature of these currencies -- the only solution is more hashing power. But the difficulty algorithm attack issues will likely be solved by fixing the algorithms.