I get the impression that this tool isn't just around for kicks, the way Google Earth is. This spreadsheet program is going to be integrated with Google's core business, search.
Here's the comment I had at Gadzooki on the issue, which will help you understand what I mean when I say Google is developing this for search.
"Google's latest development in indexing information. I think they expect this can help refine search the way Yahoo is using Del.icio.us, namely social networking. If the sheets are in the public domain, well... [it is reasonable to assume they can be indexed and used to help make numeric-related searches more accurate].
I think Google's motivation for doing this is also as a counterattack on Microsoft. They want to force Microsoft to devote time and energy to maintaining its spreadsheet software market share and developing its product, so that it can't focus as much on search."