Q:

WatermarkedTextBox for Windows Phone 7?

I am looking for some kind of WatermarkedTextBox for Windows Phone 7? But I need a custom control not how to implement Watermark behavior or how to handle the GotFocus/LostFocus events.

A:

This has been added to the official Windows Phone 7 Silverlight Toolkit, and as covered here.  It's called PhoneTextBox and can be used like:
<toolkit:PhoneTextBox Hint="Your Text Hint"/>

