Whoa! Hang on! You are getting some advice that can get you into trouble, since you don't have the full picture.
It is correct that all the transactions (both to you and from you) are stored in the public blockchain that all full nodes have a complete copy of. So as long as you have the correct private keys in your wallet.dat file, it will sync up no matter how old the wallet.dat file is.
However, what nobody has explained clearly to you yet is that the wallet.dat file has a limited number of private keys in it.
Each of your bitcoin addresses is associated with exactly one private key in your wallet.dat. However, every time you create a transaction to send bitcoins, your client also may create a special bitcoin address that it keeps hidden from you and sends some of your coins there. Those coins still show up in your wallet balance because your client stores these hidden addresses and their private keys in your wallet.dat. This means that even if you only have 5 bitcoin addresses in your "receive coins" tab in the user interface, if you have sent bitcoins 10 times, you probably actually have 15 addresses in your wallet.dat file (1 for each address in "receive coins, AND one for each transaction when you sent coins).
The client file pre-generates 100 addresses along with the associated private keys. So when you request a new address from the client (or when it needs a new address for the "change" in a transaction where you sent bitcoins), rather than generating a new one for you it grabs the next one from this pool of pre-generated addresses and then generates a new one to take its place in the pool.
This means that when you back-up wallet.dat, you are backing up all of the private keys that you are currently using AND the next 100 addresses that the client will use in the future. If you generate a bunch of new addresses and/or create a bunch of transactions to send bitcoins, you'll need to make sure that you backup the wallet.dat again before you've used up 100 addresses or you could lose your bitcoins permanently.
As an example:
User has Bitcoin-QT client and only has the original address when he starts it up.
User backs up wallet.dat.
User receives 100 BTC in a single transaction output to that address and never receives any other bitcoins to this wallet.
User performs 110 transactions sending 0.01 BTC each time
User's wallet shows a balance of 98.9 BTC
User deletes and re-installs the client, then replaces wallet.dat with the backup.
User waits for the client to sync.
User discovers the wallet shows a balance of 0 BTC, and has permanently lost all access to the 98.9 BTC.
So keep in mind that you will want to have a recent copy of the wallet.dat. It isn't enough to just have an old copy. How recent it needs to be depends on how many addresses you generate and how many transactions you send.
Also be aware that when you encrypt your wallet, it tosses out the entire 100 address pool, and generates a new set of 100 addresses. If you create a backup before encrypting, it won't be good for very long after encrypting.
+1 for an excellent post. I didn't know any of that. So it's wisest to backup a wallet, say, once a week or so to insure that the private keys are up to date?
