penders - 10:31 am on Nov 1, 2011 (gmt 0)
The <center> tag is deprecated (since it is presentational) and <p> can only contain certain other elements since it is primarily intended for a paragraph of text. The <div> element is probably the one to go for, but it will depend on the context where it is to be used. You might even be able to center the Goodle-Ad using existing elements?
You could perhaps post the resulting markup to see what we have to work with, but I suspect this is more of a CSS issue than an HTML one?