If you want to implement a grid for your Windows Phone 7 app you have to use a Listbox control because of the missing DataGrid at the Windows Phone 7 platform. A first idea for contsructing a Grid then could be to use the WrapPanel control in your ItemTemplate of the Listbox control. But if you do this, the data virtualization of the ListBox will be disabled (Virtualizing Data in Windows Phone 7 Silverlight Applications - Peter Torr's Blog - Site Home - MSDN Blogs). This will increase the loading time of your ListBox and results in a very poor UX handling. The problem is, that you can not use any complex data structures in the ItemTemplate at a Listbox for Windows Phone 7. Every line in the Listbox should be one data line. So a solution for developing a grid styled Listbox is to implement a RowAdapter class for your bounded data.
Here a implementation of a RowAdapter, I used in one of my projects: