I'm stuck, at this moment, with Firefox 3.6, so I'm not sure this is what really happens.
When I reply to someone on StackExchange, and write @u(sername), an intellisense like tag appears, floating, just above the @u text with the 'username'. This is great, of course. However, and that's where my doubt is coming from, I'm not able to accept the suggested username by keyboard. I have to click on it.
Intellisense like systems accept either [tab] or [enter] as 'accept', and [esc] as 'cancel'.
The question is if you have noticed the same behavior in more modern browsers, and if this should be more keyboard oriented?