[ryan-c]

A 51% attack will let you double spend and roll back transactions. If one wanted to steal an established name on Namecoin, they'd need to either roll back to before it's registration, roll back to before it's previous renewal and force an expiration, or mine past it's next renewal to force expiration. Renewal limit is 36,000 blocks. For a name that consistently renews early every 18,000 blocks a 51% attack would need to go on for months to steal it. Still possible, but very difficult.

Namecoin is merge-mined with Bitcoin by many miners, so if you wanted to 51% it you'd need a pretty large chunk of Bitcoin's hash power. An evil pool could do this, however it would be visible on the Bitcoin blockchain that they were doing it (although not what transactions are present/not present in the attack chain).