Historian Juan Cole (Michigan) has some interesting details and grounded speculations.  He also links to this astute analysis; an excerpt:

What we now know about the London-based plot to destroy ten  civilian airplanes points to six conclusions. 

First, what stopped this plot was law enforcement.  Law  enforcement.
Not a military invasion of Pakistan, Iran, Lebanon, Egypt, or Iraq.
Old-fashioned surveillance, development of human sources, putting
pieces together, and cooperation with foreign police and intelligence
services….

Third, if al Qaeda was involved (allegedly from
Pakistan), we can thank the failure of the war in Afghanistan and the
cozying up to Musharraf to destroy them. 

Fourth, there was no involvement by any American-based “cells,” according to the FBI Director Robert Mueller.  As many of us have been saying
for nearly five years, and as the 9/11 Commission Report showed, there
is virtually no plausible American jihad organization at work, and
never has been. 

Fifth, the plot again reveals how ill-equipped the
U.S. Government has been in anticipating plausible attack scenarios and
taking steps to prevent them.  Liquid bombs were so hard to figure out?
Al Qaeda already tried it.  DHS has almost completely missed the
threat, just as they are missing the vulnerability of cargo holds and
God knows what else. Thomas Kean, the former GOP governor and co-chair
of the 9/11 Commission, called this liquid bomb error “appalling” and
wondered, on an NBC program four months ago, why no progress had been
made.  What are the tens of billions being spent on?  This is Katrina
II.

Sixth, and most important, we must end our
involvement in Iraq and sharply refocus our presence in the region.
The war president’s approach is not working.  It’s a diversion from the
real threat.  It’s a spur to bitter revenge.  It’s a big feedback loop
that will endanger us for years, if not decades.  Our lives are now at
stake because the Bush catastrophe has created thousands of new
terrorists….

Reversing America’s colossally destructive series of
interventions in the Middle East—a cause, a trigger, a recruitment
fountain, and a charity for jihad—will require an entirely different
mindset, not just an adjustment or a measured retreat.  When America
responded, after being prodded, to the tsunami victims in Indonesia
early last year, it profoundly changed Indonesians’ views of the United
States.  New attitudes of support and cooperation suddenly sprang
forth.  This “natural experiment” should be examined to learn from,
possibly to emulate, in the Persian Gulf and elsewhere. 

We’re now viewed as destroyers, and destruction is
the retort.  This is the “new Middle East” that is aborning—one of
relentless violence—if we do not end our own relentless violence
there.  The would-be bombers in London are a reminder of how close it
is.

In the U.S., as most readers probably know, there is a ban on all liquids, creams and gels being brought on board airplanes.  An exception is made for prescription medications.  I would imagine that all would-be mass murderers will simply get a prescription, pour out its contents, and refill with appropriate explosive material.  If this simple thought  can occur to a simple soul like me, surely it must occur to someone at the FBI or DHS who is supposed to be professionally occupied with preventing mass murder.  (But the threat of liquid explosives apparently didn’t occur to them…or if it did, they didn’t think it worth taking any precautions.)  I suppose the next stage will be for all medications on board to be under lock and key during the flight.

This might be an apt time to go out and rent Terry Gilliam’s prescient 1985 film Brazil.