Typing on a smartphone can be somewhat of a challenge. Therefore as much help as possible is often preferred. When the user selects a TextBox in a Windows Phone 7 app it will show the normal alphanumeric keyboard by default.
If the user should input a numeric value it would be a help for the user that the numeric keyboard was show as default instead. The trick is to set the InputScope property. Setting InputScope="CurrencyAmount" on a TextBox will force the numeric keyboard to be show.
//xamlgeek