Microsoft elaborate on top 10 list of things to do great when designing your Windows Phone 7 app. Number two goes over "login screens" and some of the issues could be improved when creating screens with input fields.
On login and other input type screens, make sure action buttons aren't placed at the bottom of screen as the keyboard will hide them upon text field selection. Don't assume user will tap outside a text field to dismiss keyboard and reveal a "Sign In" button.
It's recommended to use platform app bar with standardized buttons for "OK", "Cancel", "Save" etc.
If you must have buttons with text, given that text buttons aren't supported on app bar and they need to be placed on screen, try to have them visible when keyboard is displayed. You can use up to two input fields on the screen, plus one or two text buttons to accomplish that.
Avoid login screens before launching the app: Even if your app is only for registered members you must allow any user to browse your app--everybody willn't know the service or product you're offering. Requiring registration before communicating the value or benefit your app is providing presents a poor experience.