Quoting Dopplex, reply 31The Beta is actually going to run on MS Excel instead of the final game engine...Funny you should say that...
http://www.excelgames.org/
Actually, I work at a company that does "Serious Games" using Excel, so I know well the kinds of hoops you can get Excel to jump through if you really try
Our uses are a slightly more traditional use of a spreadsheet though (well, at least compared to ExcelGames.org), since our "Games" are more along the lines of Competitive CEOing (with an educational focus on finance and business acumen) which involve much more of the type of modeling that spreadsheets are meant for.
When you get down to it, a spreadsheet is just a calculation engine - and since games tend to do an awful lot of calculation, there are far worse fits in the world.
Something mechanically like Elemental (or more likely something like an early iteration of Civ) actually wouldn't be undoable in a spreadsheet (although poor old VBA might get seriously overloaded...). Almost anything turn based could work quite well, actually...
(Dammit, now I'm going to spend the rest of the day trying to figure out how I'd have to go about recreating MoM's tactical combat system in Excel...)