The issue seems to impact new publishers, recently approved, for days and weeks after they received their official welcome message from Google.
The Google AdSense team emails the publisher:
Congratulations, your AdSense account has been approved to show AdSense ads on your own website. Within a few hours, you will begin to see live ads.
But even days and weeks later, their ads are still not showing.
Supposedly this is a known issue. A top contributor in the forums said the team is aware of it:
From the AdSense staff. The issue was raised to them last week, and they're looking into this. You won't be able to monitor it.
When Adsense has an answer, either one of the TCs will post it in the thread, or all affected publishers would receive a notice about it. As of yet, I don't know how AdSense will choose to inform people when it's fixed.
I am not sure on an ETA for a fix.