Just to play devil's advocate here, and I'm not for one second in any way justifying the actions of the copper in the video, but there are a few things I did want to offer for the sake of balance in what seems a mostly one sided discussion...
1. This video, shocking though it is, is not necessarily indicative of a systemic problem of violence or brutality within the plod. Nor, I think, should all coppers be tarred with the same brush. A lot of them are decent people doing a tricky job.
2. For all this talk of violence, we shouldn't forget the pointless aggression also seen on that day from the protesters(and I know it was a TINY, microscopic minority of an otherwise peaceful crowd, but still, smashing windows and burning car achieves absolutely fuck all.) With all the press hysteria leading up to it the atmosphere was pretty tense anyway, NOTHING justifies the coppers actions, but it can explain it.
3. I do get that impression that many people on the internet, this forum especially, come to the table expecting a police fuck up and then take a particular amount of glee when they do. I can't see this as a healthy attitude. Like it or not we need the police and the only way we are going to improve things in this area is through mature and rational reconciliation and dialogue, rather than an immature 'us vs them' mentality which drives the coppers into these tragic fuck ups in the first place.
just to clarify: that video is tragic, and the copper who shoved Ian Tomlinson should be tried and convicted for manslaughter, and the people who lied about the nature of his death should be sacked, at the very least. I can see what's wrong here, but it's way too easy to self-righteous in that classic lefty 'fuck the police' way, in my extremely humble opinion.
edit: while I was writing this post, agricola made some very interesting and balanced posts, so my post seems a bit unecessary, but you get my drift.