What exactly happened at the pre-summit G20 protests in London is still being sorted out. But there's a notable precedent for the police tactic of trapping protesters: the infamous Pershing Park protest in Washington DC in 2002. In that case, the police blocked in protesters -- and anyone else who happened to be around -- and arrested them all for an illegal march. The tactics achieved their intended purpose -- intimidation -- but later turned into a legal fiasco for the city. The new wheeze is apparently just to trap the protesters without ever actually arresting them.
UPDATE: One problem with the tactic of penning in protesters is that you have forcefully herd in anyone who happens to be around, like Ian Tomlinson.