My understanding is that Google is not accepting sites which contain excessive advertising, and that we can't have other companies' text ads on the same page with AdSense -- that doesn't mean we can't have ANY other PPC advertising. That would be unrealistic.
I queried Google about competing text ads right after being accepted into AdSense. The reply pointed to the Adsense policy page, but went on to indicate that it was okay to display text ads which are clearly marked and differentiated from Google AdWords.
The following is quoted from Google Adsense's general FAQ:
"If you already display Google AdWords ads on your pages via an ad network, you can still join AdSense and run ads on your site through our program. However, you cannot display more than one set of AdWords ads on the same web page. This constitutes double-serving, which Google does not support.
It's your responsibility to work out any contractual issues that may arise with your ad network as a result of serving AdWords ads directly."
I believe Fastclick is one of the so called "ad network" that "display AdWords ads on your pages." So I think there won't be a problem as long as you don't put both Fastclick and Adsense banners or skyscrapers on the same page.
the rate was appalling....I did "Google Contextual Text Ads" at Burst and the cpm was about a quarter of what I am getting doing it directly thru adsense. Google were smart to use burst and fc to test out the thing for a few months prior to the big June launch. The main ad companies have now had to put up with the ad publishers switching en masse to adsense since June.