Jeremy Likness has built a control that allows a calculator-like experience for entering numbers.
Source: C#er : IMage
I'm exploring the phone more and more and came across the case of allowing the user to enter digits. One thing to keep in mind on the phone is that the form factor requires approaches to input that are different from the traditional desktop. A desktop application might handle numeric entry by simply filtering the text box, but that assumes a keyboard-centric model. On the phone, touch is key.