I don't know of any bitcoins lost as a result but there was an incident where DropBox was not secure for a period of time a few days ago.
  - http://techcrunch.com/2011/06/20/dropbox-security-bug-made-passwords-optional-for-four-hours/

Anyone storing their wallet on dropbox unencrypted, *AND HAD THEIR DROPBOX ACCOUNT ACCESSED* during that time, would be at risk.

The problem is that the only way to know if your account had been compromised was by receiving a notice from DopBox saying so. 

If the wallet was grabbed, not only are any coins in the wallet at risk, any future coins received (over the next 100 transactions, if the wallet uses the default address pool size) would be at risk as well.

In other words, if you happened to use DropBox to archive your wallet, unencrypted -- treat that wallet as if it were compromised.  Move your coins to a new location (e.g., an eWallet provider, such as MyBitcoin) and then create a new wallet.