According to James Kirkup in the Scotsman, the latest from London is that—

the four bombers who attacked trains and a bus in London almost certainly triggered their explosives by hand, knowing their deaths were certain.
....

More than a week of painstaking forensic reconstruction of the blast sites has failed to yield any evidence of timing devices being attached to the four bombs that killed 56 people.

The finding has left detectives all but certain that the men were suicide bombers and fully intended to take their own lives in the course of murdering as many members of the public as possible.

There had been suggestions that the bombers could have been tricked into carrying the backpack bombs without realising they would explode.

Among the evidence put forward for this theory is the fact that when they travelled to London they bought return tickets and made sure their rented car was properly parked and ticketed.

But in the apparent absence of any sort of automatic detonators, the "working assumption" among detectives and intelligence officials is that the four men knew they were on a suicide mission.

I have seen this theory promoted on a number of sites and have never thought it was very compelling.

If your intent is to deliver a bomb undetected, the first thing you must do is to arrive without being stopped or checked for any reason. A well-planned operation would certainly take that into account.

So why would you buy a round-trip ticket if you knew you weren't coming back? Because to do otherwise might raise a flag.1 Why would you properly park and ticket a rented car? Because there's always the offhand chance that a policeman (or car-lot attendant) might be in the vicinity and call you over.

The last paragraph, however, of Kirkup's account leaves me puzzled—

"What no-one can explain is how three bombs could go off within 50 seconds of each other unless the people carrying them triggered them intentionally," said a source close to the investigation.

Are timers that imprecise? Perhaps so. But if so, it's rather amazing in this day and age.

I would think it would be a great deal easier to achieve this level of precision by means of a timer rather than by having each member of the group check his watch. I would also think it would be less psychologically troubling to the would-be suicide-bomber.