Protip on how to backup wallet: encrypt it then happily upload it in gmail, msn skydrive and every other email or online service you have. Since it's encrypted, no one will steal it. And no way you can lose it, even if some service happily delete your account you still have others. And except that, these online services are very reliable.
I like (and use) the above technique. A couple of other suggestions if I may. These are simply my own personal thoughts on things and I encourage people to think about things and roll their own...avoiding a monoculture is a good thing in my opinion.
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When encrypting a wallet.dat, do NOT use a password you can remember (unless you are really good at this stuff) and for the love of God, don't forget your password(s). That means keeping it/them written down in a secure place or several.
If someone is trying to break into an on-line account, there are a limited number of attempts they can try. If someone has the encrypted wallet.dat file in their possession, they can try at a much higher rate, and with software akin to mining rigs. So the password should be much better.
(If someone has a password hash from a dump stolen from some web site's database, they can use rainbow tables and other tricks to try to obtain the password. That is a different beast.)
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I also suggest re-naming the encrypted wallet.dat to reflect info about it. e.g., 'wallet.dat__sav-2_20_20110802.enc-1'. I can see at a glance that this is my second savings wallet, it contains 20 BTC, it was created today, and it uses my '1' encryption scheme.
It sorts nicely with my other savings wallets as well. I personally keep an active working wallet with spending money and don't bother to back it up at all. My savings wallets are in cold storage.
On mistake I made was forgetting to properly document the addresses of each savings wallet (so I could check the balances on blockexplorer and make sure all was as expected over time.) Had to open a bunch of them back up to get it.
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I also suggest if one is puttering around with command line tools, be most careful indeed to not overwrite or prematurely throw away a wallet.dat, and to double check that the end result can be turned back into a functional wallet.dat file when a technique is decided upon (and documented.)
Most people will want to use a canned tool for such an operation. Fine, but be careful about where the tool came from and be advised that attackers will know exactly how it works and how to exploit whatever weaknesses it has even if the tool is perfectly legitimate.
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Lastly, I suggest that deleting a file off a hard drive, pen drive, etc, is not as definitive as a lot of people think. There could be a whole lot of 'deleted' wallet.dat files to be found if one's HDD is stolen and if the person who is looking is proficient. There are programs which can securely and reliably nuke a file. Depending on your risk profile, keep this in mind.