I got this error when trying to use older wallet.dat backup with 0.6.6.2 on ubuntu
v0.6.5.x used v4.8 BerkeleyDB for block index and wallet. Recent releases make use of v5.3.28 BerkeleyDB. You probably need to export your private keys and import them into v0.6.6.2 or v0.7 beta. The latter is better for private use.