I do not appear to have .dat files on my mac.As long as you don't create new addresses, no. Wallet.dat files just store the private key associated with an address, so it's basically like an account number.
That's because it's downloading the block chain. Every time you make a transaction, it appends the block chain. So until you have the fully updated block chain to append, you can't make a transaction. Therefore, you want to leave the Bitcoin client running on one PC where you do your transactions so you've always got the fully updated block chain.
sounds good. It's been running since before I posted this and it's maybe only 1/3 complete. Is that normal? That puts the wallet running at about 5 days.