There are a bunch of ways you can use Python in excel for windows (I could never get it to work on the mac version, but I couldn't even get Solver to work on the mac version) One goes through the com the other is a plugin of some kind.

What is truly cool about this one though is that you can leave any python object into a cell. Very cool. Its like programming python in a visual functional language. I suspect that this will be abused. "Yo, look at this slick web app that is run through a spreadsheet".

Edit:

I almost forgot my favorite way to write any code in any language for Excel. Remote User Defined Functions. http://msdn.microsoft.com/en-us/library/ff475859.aspx They are bloody awesome and totally asynchronous in Excel 2010.