I work with (multi-terabyte, not multi-petabyte) GIS databases. I am also a Haskell programmer (though not for my day job) so MapReduce doesn't scare me off at all. It's very hard to see how MapReduce specifically would help large-scale GIS.
The main benefit of MapReduce for most problems isn't the programming model, it's the principle "move your code to where the data is" in a way that's agnostic to precisely where the data is. When you have big data, you need to do that. Precisely what that code does is a secondary concern.