Excel Services is such a new product that many people have trouble understanding exactly what it does, why they'll need it, and why a grid computing guy like myself is always talking about it.
Simply put, Excel Services is like Excel on SOA: it lets you take the power of a spreadsheet in Excel, then make it available as a service (either through a browser or web services) to everyone in your enterprise. As I've written extensively in the past, I think it's a game-changing product.
What does this have to do with grid? Scalability. When you make something available as a service, you've got to ensure that you're ready to have many people use it at once.
As a Gold Certified Microsoft ISV partner, I was tasked to make a 5-minute video for the Office 2007 launch explaining how a grid can help ensure scalability for Excel Services and UDFs. If you're interested, check it out here on the Microsoft site. Read all about it, or scroll down and click the video link. (Note: the video link worked for me in Firefox, but not in IE 7)