Google posted on the AdSense known issues page a new issue around new AdSense publishers seeing ads on their web sites too soon.
The bug was published late last night or early this morning (EST time). It reads:
Some new publishers are briefly seeing ads after signup
Some new publishers have reported seeing ads on their blogs immediately after signing up for AdSense via a host partner site, but before their account has been fully reviewed. The ads stop showing after a few hours. Our engineers are currently investigating the issue and working to resolve it as quickly as possible.
Ads should not show until the new AdSense accounts are fully reviewed. Of course, I do not see complaints from anyone on this. But if you recently signed up for AdSense and see ads immediately, it is a bug.
Google did expedite the AdSense sign up process, but this seems unrelated to the bug.
Forum discussion at Twitter.