HTTPS is one of the most secure ways of sending and receiving content on the web. Many sites including Gmail and several other Google products make use of the HTTPS protocol, even Hotmail does it too.
However, it looks like Twitter has now rolled out a new setting which allows users to Always user HTTPS. To enable this setting you can go to your profile and edit settings (or go to http://twitter.com/settings/account) and check the box next to “Always use HTTPS”.
If you think that HTTPS will slow down your experience, you might want to read an earlier post on how much impact Google saw when it shifted to using SSL.
Update: This feature seems to have been pulled off by Twitter now.