I'm talking with another agency about reselling your services to them, but obviously it doesn't make sense for their clients to see my logo. Is it possible or could it be possible in the future to adjust the header and login logo based upon the client? Obviously the login logo would depend on the client using a custom domain.
Although it's possible to setup a custom domain for each client, I'm afraid you cannot change the logo or header on a per client basis currently. This is on our feature request list however, so I'll get a vote added on your behalf - we'll keep you posted on any updates.
In the meantime, perhaps you could setup a new agency account for the other agency, so it's branded to them instead.