How is wallet at official client encrypted? What encryption it use?
What encryption does Bitcoin-qt use and what encryption is used at Multibit? And how strong that encryption is compared to other methods?
Both bitcoind and bitcoinj use AES-256 encryption for the private keys in their wallets (this does not mean that the wallet formats are compatible though).
Bitcoin-Qt is just a GUI on top of bitcoind and Multibit is using bitcoinj internally, so the same should apply to them.