At least, judging by the photos I've seen (http://cdn-static.zdnet.com/i/story/70/00/001744/berners-lee...) the monitor was shining bright yellow, when in reality they are greyscale monitors with a bluish cast. I think the monitor was a shell, fitted with a lamp to illuminate TBL from that side.
Perhaps the keyboard was wired up to a modern laptop or tablet, or some custom system tied in with the LED system at the stadium. Most likely, it was automated for precision and correctness, and he was just miming the typing.
Which is fine, IMHO, it's good stagecraft. Voldemort and the parachuting Queen weren't real either. The rest of the light show would have been preprogrammed, so it would make sense to do the same with the message written on the LED system.
But it was definitely a Cube, NeXT keyboard, NeXT mouse, and either a modified NeXT monochrome monitor or a close facsimile. There's a lot to be said for that, given how much easier it would have been to use, say, an iPad than a full Cube system - and an iPad would be vastly more recognizable to the audience, as well.
Its because she eats McDonalds and drinks Coca-cola that she has the physical prowess to pull off such a stunt.
I know, I'm British and have been paying attention to the carefully crafted messages from our Olympic Overseers for the past year. Open your eyes and you too can learn the truth...
I'm pretty sure it wasn't the actual NeXT he used. That is in a museum at CERN (where I work at the moment). I can check today by going to the museum, but it was there a few days ago when I showed some visitor friends around.
another cool things, the NeXT Cube was live on the web at the time and he tweeted from it.
Edit: someone just replied to me on twitter and mentioned that it was actually the Cube that he used at CERN, taken out of a museum for the ceremony.
This is the tweet that was sent out during the ceremony while he was on stage:
https://twitter.com/timberners_lee/status/228960085672599552
although it says 'via Twitter for iPhone' :)