We've had inline tagging as a moderator-only feature for quite a while, and it's been fairly stable, so we decided to roll it out to all users with 10,000 reputation or more.
If you have the requisite 10k reputation, just mouse over the tags on any question page, and an "edit tags" menu item will appear. Click it, and just the tags will become editable.
This is the standard tagging UI, except loaded inline rather than on a separate editing page.
Enjoy!