Google Analytics has made a subtle change to their graphing last night. When you view your Google Analytics data but limit the date range to show just one day, Google will default the view to show the "hourly" filter.
Rakesh Singh notified me of this last night on Twitter and after asking a bunch of Twitter folks, it seems that the new part is not the "hourly" option but rather it defaults to the hourly option on a specific date, like Rakesh Singh said.
Here is an example:
As you can see, I get most of my traffic (at least from yesterday) in the morning.
Google had the hourly option since at least the beginning of this year based on my research. When the new Google Analytics first launched, hourly was missing.
Forum discussion at Twitter.