I set up a new wallet for this client. Starting balance is zero, so it can only be a net win regardless of what the code wants to do.
Stealing your BTC isn't the only thing a closed-source client might do. Make sure you don't have any personal information stored on the computer running the client, no bank account numbers etc.
And monitor the network communication to make sure it only communicates with other bitcoin P2P clients, and not other botnets as well. It would be too easy to hide a key-generation botnet client inside something appearing to be a bitcoin client.
Wow, that's paranoid, and rude. Dude, we are not talking about some shady third party, you can ask the programmer the how and why right here. Nor do you have to trust him or use his code. You could do it, and open source it if you like, if you have the skills. For the time being, however, the code belongs to he who wrote it, and he can dictate the conditions. The client is open, but for now, the gpu code is not. If you want to help to make it so, someone could make an offer of a number; after which the code is open sourced by the author, whether that number comes from the gpu client or from regular donations.
How about 50K bitcoins from all sources or one year after the release date, whichever comes first?