Hey, since this is the "new" thread (of which I still consider myself), why would you need a username/password for solo mining? Where are you signing up to to confirm it's you? I'm in a pool right now, so the u/p scheme makes sense.
Mining clients which use the user/password functionality, instead of just an address which you own, are able to be run over networks. Thus you could, for example, run the mining clients on many computers connected via LAN but have them all connected to just one computer that runs the normal bitcoin client, minimising internet bandwidth use and the cpu load of verifying transactions (which is significant once you scale up). You are still "solo" mining because you're not connected to a pool, but this allows you to sort of form your own pool.