Scientists seem so certain that the recent activity at Mt. St. Helen's
will not result in a repeat of the deadly blast of 1980.  How can they
know this for certain?

I haven't heard anything that sounded like certainty on this matter
from scientists quoted in the media. Most of them have used terms like
"not likely" to describe the chance of this eruption being similar to
the one of 1980.

The last eruption was so ?deadly? because something like 50 poor souls
died. I think we might have learned from that and keep a more
respectable distance this time, so even if it was to be a bigger
explosion it wouldn?t be regarded with such horror. As I?m sure you
know, the devastation was caused by the side of the mountain
exploding. Since that area is believed still to be the point of least
resistance, the overburden being less, pressure will be relieved more
easily. A few hiccups are better than one almighty belch.

Another factor, in common with many volcanoes, is that the cycle is
reasonably consistent because it takes time for the magma chamber to
refill. One wouldn?t expect major activity after such a short pause.
However, magma chambers can split and any (surviving) volcanologist
will tell you that it doesn?t matter how much you know about
volcanoes, they can still bite unexpectedly.

Best

Vulcanologists face a dilemma - if they say nothing's going to happen,
and it does, people might die.  If they say something will happen, and
it doesn't, there's a lot of (very expensive!) disruption and they're
accused of crying wolf.  The next warning's ignored and people die! 
Like pinkfreud, I haven't heard definitive statements - rather,
they've been intended to get people into the right mindset so that
they don't worry too much but they'll jump when they have to.

Ian G.

Sometimes I ask my self, if it would not be better, to "Bomb a hole"
at a side of the volcano where there are no people.

then the pressure which is inside could be released to that side, and
an explosion would be prevented. Also, the people could influence the
"time" and would not have to wait for something.

Don't they have the technology to make a hole into a volcano (from the
backside) so his pressure could be released before it explodes?

They can't.

Don't overlook the role of water in the devastation caused by volcanic
eruptions.  In 1980, St. Helen's was covered with glaciers.  Upon
eruption, the solid water (ice & snow) quickly becomes vapor & liquid
leading to lahars, which are often one of the most "deadly" mecahnisms
of an erupting volcano.  This time around, there is much less snow and
ice on top (St Helen's is actually home to one of the few glaciers in
the world that is actually growing though).  Check out USGS websites &
links on St. Helen's for more info:

Lahars: 
http://volcanoes.usgs.gov/Hazards/What/Lahars/SnowLahar.html
http://volcanoes.usgs.gov/Hazards/Effects/MuddyLahar.html

Cascades Volcano Observatory:  
http://vulcan.wr.usgs.gov/