The change occurred yesterday, when the Chrome Dev channel was moved from version 11 to version 12. We're not yet sure whether this is simply a mistake, or an actual reversion to the previous logo. We're seeing the change on both Windows and Mac builds of Chrome, incidentally.
The odd thing is, it was only four days ago that the Google Chrome team published its rationale for the previous logo change. It sounds like they put a lot of effort into the new logo, too! If the logo has actually been reverted, it's possible that the new logo was merely a beta test -- Google may have guaged populist opinion and found that people preferred the orb.
Which logo do you prefer? You can see some before-and-after(-and-before) shots of the Chrome logo after the break.
[thanks to Matthew for the tip!]