First sub-topic:
The sequence of events surrounding the liquid ban is unclear to those without the need to know, but AIUI it was the Pakistanis who precipitated it by announcing they had arrested one of the alleged conspirators. The British intelligence service(s) had the UK players under covert surveillance and had done so for some time; and wanted to continue. Who made the actual decision to panic err... sorry, react is not exactly clear but it seems not to have been DHS, since Commissar Chertoff was apparently hauled from his bed in the middle of the night to be told what had already been implemented in the UK.
Second sub-topic:
Whether the TSA screening has in fact deterred any attacks is known only to any such putative attackers. I think those who "rant" about the screening theatrics argue that a repeat of a 9/11 style attack is virtually impossible due to a) fortified flight decks and b) likely passenger reaction. Whether terrorists would be willing to settle for a lesser attack consisting solely of mayhem with knives etc., again only they can say. My feeling is that 9/11 and the subway attacks have set the bar higher than that.