Simplejson was slightly faster on two out of three tests. Consistently so, when I re-ran the tests.
Test environment: py2.6 on Mac OS X, with simplejson 2.0.9 and python-cjson 1.0.5
Test script: http://gist.github.com/72413
Also, I changed the test script from using time.time() to time.clock(), which according to the python docs should be used for performance testing on unixes.