Twitter has announced that they are now shortening https links with their own t.co link shortening service on https.
The company had planned to start doing it earlier but the implementation took time and it is finally being rolled out.
A company developer announced:
As we announced awhile back, we’re going to begin wrapping HTTPS-based URLs with HTTPS-based t.co links, though a bit later than anticipated. If you’re already using the recommended GET help/configuration’s short_url_length_https and short_url_length_http values for calculating URL lengths, you shouldn’t notice any difference. Effectively at the current values, HTTPS-based URLs will account for 21 characters while standard HTTP-based t.cos will continue at their typical 20 character length.