1. I don't go " online" for several months? Would I still recieve payments made, and does the BTC client just " catch up"?
Yes. If someone posts a transaction to you, that transaction is part of the public record forever. You can claim those coins at any time.
2. I loose my PC wallet (harddisk crash) and I forgot to update my wallet.dat on the USB? Does my Balance revert back to date of backup?
It depends. In most cases, your old wallet.dat will contain precisely the same keys as the new one, so you'll have your full balance. However, it is possible that your backup wallet will be completely empty. For example:
1) I get 100 bitcoins to account A. I backup my wallet.
2) I transfer 1 bitcoin to account B. My client creates a transaction that imports the 100 bitcoins from account A, export 1 bitcoind to account B and exports my 99 bitcoins of 'change' to a new internal account.
3) I lose my wallet, my backup only has account A, which now has 0 bitcoins.
The client actually creates accounts by the hundred, even though it doesn't show them all to you. So odds are the keys the new wallet used are in your old wallet anyway.
3. I loose my wallet completely?
Coins are lost forever.