They do seem to do things somewhat differently in the US, with
splitters instead of riving knives - and it does seem very common to
leave the crown guard off there.

Don't believe everything that you see done in the woodworking shows on the TV.

I'm basing it on this http://www.woodworking2.org/AccidentSurvey/search.htm
and on what've read on rec.woodworking - it would be rare to hear of
UK woodworkers disregarding crown guards, but I hear it over and over
again form US woodworkers.

In the UK (and I believe the rest of Europe) properly adjusted riving
knives and closely fitted crown guards are mandatory in the workplace
- along with a ban on dado sets and severe restrictions on deep sawing

The requirement is that the tooling can be stopped in under 10 seconds
and that it is properly guarded.

Many older machines are incapable of doing this.

Yep, my apologies - what I said is only code of practice:
http://www.hse.gov.uk/pubns/wis16.pdf

AIUI older machines that can't meet the guarding and 10 second rule
(in the workplace) have to be modified or replaced as various aspects
of the legislation cuts in (can't quite remember where we're up to -
not sure if the 10 second rule applies to all machines yet)

It doesn't eliminate risks here - and if could buy sawstop built in or
have it retrofitted at a reasonable price, I'd buy it-

I wouldn't. This is not a substitute for proper guarding, just a last
ditch stand for when all else has failed.

Absolutely. Safety is about safe working practices (aka accident
avoidance/primary safety). And last ditch (aka secondary safety/
minimising the consequences of an accident) features when the day
comes I or someone else makes a very bad mistake - so I have no
objections to hardhats, RCD's and airbags in cars either.

I wonder if sawstop technology could be adapted for spindle moulders -
as they have an even more fearsome reputation for stealing fingers
(even with the new limited tooling)?