@lbugnion offers keen insights into development for touch surfaces, including optimal screen size, using transparent backgrounds, and negative margins.  Check it out at his blog here!

Excerpt:

• Remember to maximize the touch surface for your controls.
• Plan your design in consequence by reserving enough room around each control to allow their hit surface to be expanded as shown in this article.
• Do not cram too many controls in one page. If REALLY needed, use an additional page (or even better: use a Pivot control with multiple pivot items) for the controls that don’t fit on the first one.