Updated: Not quite, looks like this was part of the limited launch for "Google+". https://plus.google.com
Google appear to be testing a new white-on-black navigation top bar, at least in the UK. Or, at least to some users.
Google have the kind of usage numbers that can make multivariate testing (or A/B testing) incredibly insightful. Google frequently test little tweaks to their user interface and I'd love to see the data they got back from this trial.