Even if an attacker obtained my wallet.dat, if it is encrypted with a long password, is it safe to say the stolen wallet is, for the time being, useless to the attacker?
Yes it is useless to the attacker.
But in a worst case scenario with the hearbleed bug, however, if you just recently sent coin and therefore unlocked your wallet, the private keys might be in memory and leaked to the attacker. This is just theoretical, in practice probably very unlikely.