The pins in Excel 2010's File,Recent action allow you to keep certain recently-used workbooks at the top of the Recent-Workbooks list. This means the workbooks you use a lot won't fall off the end of that list, during some week when you were doing a lot of work on something else; later on it'll still be there.

But that doesn't help you keep the Excel window on top of all the others in your screen. If that feature exists, it may be called "pinning" but it's not the same as the "pinning" that's done with recently-used workbooks.