Google's services don't usually have interfaces optimized for tablets. They either use the desktop version (Google Search) or use the mobile version (Google Calendar, Google Docs). An important exception is Gmail, which started to test a tablet UI shortly after iPad's launch.
Google tests a new homepage and a new search interface for tablets. Unlike the standard desktop version, the new UI places the navigation menu and the search options sidebar at the top of the page, so that the search results take up most of the space.
Amit Agarwal, who first spotted the experimental design, says that it's cleaner. "The new Google design uses a single column layout, while the old sidebar options appear between the search box and the search results. There's plenty of whitespace between search results and links to the Cached version of pages have been removed in the new design."
The new layout is more readable and it's also used for specialized search engines like Google Image Search and Google News. Google Image Search for iPad now uses infinite scrolling and shows a lot more results. Unfortunately, the list of missing feature is impressive: no Google Instant or Google Suggest, no links to the cached pages or to the mobile transcoder, no link to the advanced search page.
I've only seen the experimental UI in iPad's mobile Safari, but I'm sure that it should also be available for Android tablets like Motorola Xoom, Samsung Galaxy Tab 10.1 or Asus Eee Pad Transformer. Clearing browser's cookies might help.
Update: Google Mobile blog informs that the new UI "is rolling out in the coming days to iPad and Android 3.1+ tablets across 36 languages."
{ Thanks, Mushaf. }