As we reported last week, the Google cache was going HTTPS but I wasn't able to consistently bring it up. It now appears that all HTTPS sites will be rendered in the Google Cache under HTTPS Google Cache URLs.
If you search for this site in Google by way of [search engine roundtable] and click on the Google Cache, you will see the HTTPS URL by default:
I suspect last week Google was testing the waters with it and now it is officially 100% live for all HTTPS sites. The move does make sense.
Gary Illyes confirmed on Twitter that this fully rolled out this past Monday.
Forum discussion at Twitter.