DigitalPoint Forums reports on the What's New in April for Google AdSense. Google added two new ad formats, (1) Square (250 X 250) and (2) Large Rectangle (336 X 280) for ad format options. And made site-flavored search smarter by "learning technology, site-flavored search delivers search results that improve over time to become customized to the interests of your visitors."
Overall Google AdSense Publishers in the forums are happy with these additions.
Forum discussion at DigitalPoint Forums.
There is also more detailed discussion about the mysterious changes to the site-flavored search at WebmasterWorld.