@Hawkee I can tell. :) Before it took quite some time for the autocomplete to popup. Also, nice on the avatar addition.
@Abcdefmonkey I still have some more adjustments to make to it. I plan to cache the names most likely to be mentioned so those will come up instantly.
@Firstmate There is a very good reason I did it this way. If you @mention somebody who has a space in their name it would be very difficult to parse the text to convert it to a link. For this reason I have a hidden field that mirrors the text as you type except the username is encoded with the user_id. So when this is saved to the database it's actually saved like this: @ [47706]. In order for this to happen you need to select the name from the autocomplete dropdown. This is how it works on Facebook and Google+. Only Twitter parses the actual text of the post and they can get away with it because there are no spaces in their usernames.
@Hawkee nice one :D