How to convert tag-and-username-like text into proper links in a twitter message?
I'm not sure exactly what you're asking, but what's wrong with something like {{ user.get_absolute_url }}? For the tag detail URLs, it really depends on what you're looking for, but you would have to construct the url and view for that yourself.
I'm writing a twitter-like note-taking web app.
In a page the latest 20 notes of the user will be listed, 
and when the user scroll to the bottom of the browser window more items will be loaded and rendered.
The initial 20 notes are part of the generated html of my django template, but the other dynamically loaded items are in json format.
I want to know how do I do the tag-and-username converting consistently.
Thanks in advance.