via Tumblr staff:

Today, we’ve added two new ways to use tags:

Tags can now be displayed for each post in your theme (see the Custom Theme reference’s new Tags section). You can link to URLs that look like this:

http://tumblelog.marco.org/tagged/New_music_experiment

…where you can replace New_music_experiment with your tag name. Underscores in the URL are interpreted as spaces in the name, so that tag was actually entered as "New music experiment".

The standard view shows posts in reverse-chronological order (latest posts on top), like your tumblelog’s front page. If you want the opposite (earliest posts on top), you can append /chrono to any tag URL to display the posts in chronological order, like this:

http://tumblelog.marco.org/tagged/Ohio_trip/chrono

Наконец-то! Давно я этого ждал и оказывается не зря я тегами каждый пост помечал…