People have already lost coins, but since the implementation of the key-pool I haven't heard of any unsolved problems.
I lost some partially due to my own stupidity, partially due to not realizing that this change
Technical note: the encrypted wallet’s ‘keypool’ will be regenerated the first time you request a new bitcoin address;
I keep multiple wallets, encrypted with GPG. I have a script that lets me choose a wallet, unpacks it to the ~/.bitcoin directory, waits for Bitcoin to close, and then encrypts and securely deletes the wallet. For this reason, if something goes wrong, I feel comfortable deleting the wallet.dat file since I have a very recent (encrypted) backup.
So, I updated to the new version, it futzed around with my wallet, I sent some coins to myself (change going to an address from the (fresh) key pool). Then I closed Bitcoin and something went wrong with my script, so I deleted wallet.dat, along with the private key used for the change. I lost 77 BTC that day and it has made me much more cautious.