It doesn't look like the desired capabilities flag is working when opening a chrome browser. I think it might be related to this change in the selenium library.
http://code.google.com/p/selenium/source/diff?spec=svn18044&r=17342&format=side&path=/trunk/py/selenium/webdriver/chrome/webdriver.py&old_path=/trunk/py/selenium/webdriver/chrome/webdriver.py&old=17055
Using keyword Open browser as follows:
Open browser ${url} browser=chrome desired_capabilities=--enable-logging:,--v:1
And then in the log:
DEBUG POST http://127.0.0.1:49881/session {"sessionId": null, "desiredCapabilities": {"platform": "ANY", "browserName": "chrome", "version": "", "javascriptEnabled": true, "chromeOptions": {"binary": "", "args": [], "extensions": []}}}