That is correct according to Google's robot.txt checker. If you ever want to test how Google will respond to your robots.txt you can use their tools in their Webmaster Central area.
Note that it's just my preference to put 'bot-specific directives before catchalls, and I don't use the Allow as it's non-standard. In this case, of course, the Mediapartners bot would obey it; I just don't want to keep track of which 'bots obey it or not.
I have the same issue and I just followed the guidelines above. How long does it take to show results? I just launched a website and initially I just blocked Googlebot but then I did not see any AdSense ads. Now I have allowed MediaPartners-Google and when can I expect to see ads?
