Everything is accessed though the OS, though the appropriate hardware layers. Chrome 10 and Firefox 4 also use assistance from the GPU. There are many ways to use a GPU, for example letting it do compositing (graphical) or computations through OpenCL/CUDA/DirectCompute etc.
IE9 just happened to be first to have it in a final version of their browser, the others are working on it as well. GPU assistance is being used in lots of computing intense software.
The impact will vary with the specifics of how it is implemented, it can make all the difference, or none at all. Whether or not a browser is GPU accelerated is insignificant in itself, it is the overall browser speed in actual use that matters.
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