Is there a way to enter a username or password by voice?
Google's Voice Search only interprets entire words or phrases but I couldn't find any way to dictate letter-by-letter to an input field.
Is this at all possible in Android?
You can spell out words with the voice input, but it puts spaces between each letter. I can't see there being a feature to support this as saying your password out loud would defeat the point of having a password.
Now if you follow the "pass-phrase" model, where your password is something like "i walk on the beach to relax", then it may work. but again, I don't think it would be a good idea to say your password out loud anyway.