Yep, the wallet file is totally os independent. You can have the same wallet.dat under windows/linux/mac/etc and each of the clients will access the same bitcoin wallet. Each time you move the wallet file though, you will have to re-download the blockchain.
Another question; if I were to transfer the same Wallet.dat to seperates client the balance would "act" the same; in other words while connected to the network the balance would be simultaneously the same or no?