It might not be a hard percentage, it might have some inherent bias, and the error margins might be quite large, but I still find AdDuplex's monthly numbers on the mobile scene a fascinating look at the smartphone world. For January, they've taken a look at the Windows Phone 8 handsets that have apps running their ad-serving technology to find out how many users have updated to the latest release of the OS... and they're reporting 54% of WP8 devices are running 'Portico'.
Now, the biggest caveat to all of this is the sample size - just 294 handsets - which give it an error margin of plus or minus 6. I think it's fair to say that the adoption rate of the new OS is still less than the impressive updates that Apple manages, but far ahead of uptake on Android devices.
Windows Phone 7.8's uptake is less, currently at just 16%, but this is perhaps understandable with the staged roll-out of the update and certain carriers holding back the update for further testing.