I spotted this test via a DigitalPoint Forums thread, where iBrowseHere.com found Google testing a Google AdSense ad with a "show more" link. Here is a screen capture:
When you click on the "show more" link, a new window opens up with more ads related to that ad, said iBrowseHere.com.
Like the other test I spotted this morning, I cannot replicate this one either.
Forum discussion at DigitalPoint Forums & WebmasterWorld.