If by importing the wallet.dat (private keys) means swapping them to other private keys on blockchain.info servers then the imported wallet will be useless but If someone puts his hands on the wallet.dat on the servers on blockchain then cracking it would be possible. wallet.dat contains a lot of information's[1][2][3] and there is some tools to read them[4] so I assume It's possible to read the addresses + balance without the need of cracking it, after having the addresses that contains a lot of BTC , the cracking begins process begins.


[1] https://en.bitcoin.it/wiki/Wallet
[2] http://bitcoin.stackexchange.com/questions/3173/what-information-does-a-wallet-contain
[3] http://bitcoin.stackexchange.com/questions/853/what-do-the-different-dat-files-contain/854#854
[4] https://github.com/jackjack-jj/pywallet