Put this wallet.dat file on a USB drive in your safe or mail it to your parents. Burn it to a CD and put it in a bank safety deposit box. Put it on a different computer. You can even email the file to yourself. Better yet, do two or three of the above. You only need to back up the wallet file once at the beginning (you don't need to do it every day or week, etc), and you should do it before you've received any money. Back it up, keep it safe, and the likelihood of you losing your Bitcoins will be lower than you dying in a car crash. If you don't back it up, the likelihood of you losing your coins is almost guaranteed.
Um... isn't this wrong (unless you are using a deterministic wallet). Every time you create a new address for receiving, or for change when sending, the wallet.dat gets a new keypair added. If you don't backup after this, then that money is lost (in the event of failure).
Wallet.dat needs to be backed up with every new transaction, or at least when enough transactions have occurred that it the loss would matter to you.
To me, I find this is the biggest problem with the software. When you do a lot of transactions, you need to make backups frequently. Practically this means that your backup device is likely sitting right next to your computer (not in a cd mailed to ma and pa). If my house burns down, my bitcoins are lost.
Now, I might be ignorent of a recent change to the software, but that was my original understanding of how it worked. Maybe it is smarter now and allocates 10K addresses for itself so that you don't need to be backing up all the time.
