1. The wallet is sacred. Everybody, who has access to the wallet, can spend your coins. The wallet is the only thing you have to keep private, and you failed ...
2. Dropbox is evil.
3. Dropbox is known to be insecure.
I couldn't agree more..
Best bet is to do as others have said..
1) Rename your wallet.dat > "drivers.dat" (or to any other none obviously named file). maybe even change to file extension from *.dat" to "*.wav" and change it back when you need to make backup
2) Package this file up into a protected archive file (Password protected) (WinRAR, etc)
3) Save it to an encrypted partition on your HDD (Tools like TrueCrypt will do this for you)
4) Save it to several different "Physical" locations i.e Not on a cloud based server. Stick it on a thumb-drive Packaged, Pass-protected and encrypted
5) next create a second wallet to use as your "pocket money" wallet, then transfer a couple of coins into this wallet. Now with your other "Main" wallet this can act like your main bank account which holds the majority of your coins. and keep this secure & offline.
6) create some IDS rules on your Security product to "Disallow" access to remote machines for the given file name in step 2 (above) block certain fie-types (although with this IDS based method issues may arise when it comes to syncing your wallet(s)) - just an idea for added security.