That kind of thing does happen when the stakes get high enough. There are a lot of rumors (and some evidence) of special-cased "cheating" on commonly used benchmarks, including the 3dMark GPU benchmarks, SPEC CPU benchmarks, even SunSpider JS benchmarks.
Both Nvidia and ATI have been caught cheating at graphics benchmarks. Their drivers would detect when the benchmark programs were being run, and go through lower quality fast paths. You can google "quack.exe" for details on one particularly famous case.
