There appears to be a number of people who use Dropbox or other similar service to backup their Bitcoin wallet.

The "Securing your wallet" wiki page ( http://en.bitcoin.it/wiki/Securing_your_wallet ) suggests that you might use a service like Dropbox for storing an encrypted archive that contains your wallet.

However, it appears many are using Dropbox to do a real-time backup of the (unencrypted) wallet.dat file.  

There are risks to doing this.

A few days ago Dropbox was under the spotlight for having security issues.  Coverage by TNW (TheNextWeb) summarized their report with:

Anyone with access to your unencrypted wallet.dat could grab that file and use it, immediately or at any time in the future to spend your bitcoins.

How might this happen?  
- Does your Dropbox account use the same username (e-mail address) and password that you used with any other site or service?
- Do you have Dropbox installed on any other device using the same account?  Your wallet.dat will "sync" / be copied to each target.
- Are you using anything less than a "strong" password?

Here's a good article on Password Security:
  http://www.infosecisland.com/blogview/12645-Check-Your-Password-Security.html

Recently I learned of an instance where Dropbox was being used specifically to keep a single wallet.dat synchronized between two different installations of the bitcoin client.  The Bitcoin client was not designed to accommodate multiple installations like this, and using Dropbox in this manner will likely bring lost bitcoins or other grief sooner rather than later.  So that this message isn't overlooked, pardon me for "shouting":

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*   PLEASE DO NOT USE DROPBOX TO SYNC YOUR WALLET BETWEEN COMPUTERS    *
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Related:
  - How to set up secure bitcoin savings account in 14 easy steps  
    http://bitcointalk.org/index.php?topic=5194.0

I agree but putting it on dropbox unencrypted is not needed and can only increase the chance someone gets it.

I use Dropbox and I love it, but I never really trusted them for privacy because they ask you to take their word for it that it's all encrypted, whereas we know here that the proper way is to never ask anyone to take your word for anything but to prove it with open source and strong crypto. That's what make Bitcoin work.

I think the way to use Dropbox to handle a wallet.dat file is to have it encrypted with GPG on the disk. That it isn't is an inexplicable design flaw.