An easier question to ask than it is to answer.

As with airbags in all vehicles, deployment criteria is not entirely based
on one parameter, but rather a dynamic combination of several.  For example,
what would make an airbag deploy in one vehicle going 25mph during an
accident and not deploy in an identical vehicle going 25mph in another
accident?

Here are just a few of the parameters:
Overall rate of decelaration.
Duration of decelaration.
Angle of impact.
Outside forces (speed of the other vehicle in a head-on collision, for
example)
Some measure the degree of braking before the impact.
Some may take into account the weight of the person and perhaps the
position/proximity of the seat to the steering wheel/dash.
Some vehicles have higher thresholds (for example, 4 wheel drive trucks,
which are expected to be used on rough terrain).
Some vehicles have lower thresholds (for example, light-weight sub compact
cars, with low mass and small passenger compartments).

Anyone who gives you a fixed figure for any vehicle (like 30mph) is just
blowing smoke at you unless they also tell you the exact parameters of the
incident as well (like "head-on collision against an immovable wall at
exactly 90 degrees impact angle with a 150lbs driver who is restrained and
has the seat all the way back and didn't apply his brakes).  Even changing
the angle of impact by only a few degrees at exactly the same speed can make
a world of difference.

In 20 years on the job including 16 years at my current department, I have
heard a great many people say they couldn't understand why the airbag did
(or didn't) go off in their accident.  Funny thing is they were all alive
and well to be able to ask that question.  The very few who were involved in
accidents where the airbag didn't help were situations where nothing would
have helped (children in reverse-facing car seat are a different story,
however).

Every time airbags are mentioned on any newsgroup it usually starts a
(sometimes heated) discussion.

Cheers - Jonathan