The only two clients that I can think of that meet those requirements right now are Electrum and MultiBit.
You might also consider using the web hosted wallet at
https://blockchain.info/walletWith blockchain.info the wallet that they store at the website is encrypted, meaning that only the person with the encryption password (you) are able to spend any of the bitcoins hosted there. They have the ability to configure the wallet so that it sends you an encrypted copy of your wallet every time you create a new address. This way if they ever cease to exist, you can decrypt the wallet and load the information into some other client and still access your bitcoins.
Now we're talking. That sounds quite cool. And the backups out of blockchain.info are easily imported to any other client including the standard bitcoin-qt?
Are there any drawbacks to blcokchain.info and electrum and multibit for that matter?
How do we know that blackchain.info isn't a giant trojan that's collecting passphrases and waiting for the right moment when they have millions of dollars worth stored there to abscond with? I know it may be a ridiculous question but in matters like these I think skepticism is healthy.