A problem with SharePoint 2007 Excel Services was that it could not query SharePoint lists.
Does SharePoint 2010 now contain this functionality?
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Alex - I still can't find any official documentation, but I've got ECS running and a SharePoint list is still not one of the options. I suppose a roundabout way of doing it could be to make the SharePoint list an external list against a SQL Server table, then use ECS against the SQL table. This would be, IMHO, opposite of why we wanted external lists in the first place though.
You could take a look at OfficeWriter from SoftArtisans. It has the ability to create and modify Excel (and Word) documents programmatically without Office installed. It also integrates with SharePoint to allow you to import SharePoint lists into Excel, which can then perform calculations on the list data. There's more information about it on their site at http://officewriter.softartisans.com/officewriter-434.aspx.
Full Disclosure - I work at SoftArtisans.