I've made some improvements:

- [bugfix] At some pages, the button would not be added: Tag wiki and pressing "Add another answer"
- [feature] selected text will now immediately be converted to an APT link
- [feature] URLs like `http://packages.ubuntu.com/package` will now be converted to `package` (technically, it takes the part after the last `/`)
- Code cleanup: stripping `http://` made no sense and was possibly a leftover from copying code directly

Download:

- http://www.lekensteyn.nl/files/insert_apt_link.user.js