Yup it's compatible, but if there was ever a significant change in the wallet format or anything like that, then it would perhaps not work with older versions.
Someone correct me if I am wrong.
EDIT:Unless you are talking about alternate clients, such as Electrum, Armory in which case I've no idea.
Backups of wallet.dat from older revisions can be used with newer clients. The reverse may not always be true.
Backups of wallet.dat are generally no compatible with alternatives to the Bitcoin.org client. The private keys from the wallet.dat can be exported without needing the Bitcoin.org client (using a tool such as pywallet) or with Gavin's bitcointools utility. Those keys can then be imported into most alternative clients.
Mt. Gox has an API call that will allow an entire wallet.dat to be imported and funds swept into that user's Mt. Gox account.
