From the pictures it looks like the cracks may only be in the stucco, and the underlying structural wall may be unaffected. It's hard to tell without actually inspecting the wall, but my guess is that these cracks are only skin deep. However, you will want to treat them to prevent them from spreading.
Here are some excerpts from the article.
All buildings are subjected to a
variety of stresses in various forms
and this stress is often transferred
to the plaster stucco membrane.
Stucco
is not and never was intended to be a
structural material.
Cement stucco is
a relatively thin, brittle shell over
a framed structure and can crack if
the stresses are transferred and
overcome the strength of the plaster
membrane.
SHRINKAGE STRESS
All Portland cement
shrinks as it cures. Controlling and
minimizing that shrinkage is critical
to minimizing cracks
Shrinkage cracking
tends to have a map-like cracking
pattern.
OUTSIDE SOURCES
Structural loads,
lateral force racking, thermal
changes, wind, lumber shrinkage and
seismic events are all examples of
outside sources of stress.
HOW HARMFUL IS A HAIRLINE CRACK?
a hairline
crack does not automatically mean the
stucco is faulty, failing or will not
function for the service life of the
building.
MAKING REPAIRS
Once the decision is made to make
repairs, there are several options.
Option 1: Treat the joint with the
same finish material.
Option 2: Paint the wall or ceiling
after treating the crack with an
acrylic paint or paint designed for
stucco.
Option 3: An elastomeric paint
coating.
Option 4: Recoat the wall from
architectural break to architectural
break with the same finish material.
Option 5: A new basecoat and mesh over
the wall area.
Best Answer
I don't own a house with plaster so please take this advice with a grain of salt...
As you pointed out, the cracks usually appear due to settling. As a result, you ideally want your repair technique to be resilient against future movement. As far as I know, there are 2 main ways of doing this:
Method #1: For smaller cracks, fill them in with painter's caulk (latex caulk). Because it's a latex compound, it will (supposedly) expand and contract with the crack. It also makes cleanup a breeze.
Method #2: Use a joint compound specifically made for plaster work, tape over the crack, and apply additional swipes with larger blades to smooth the look. The technique is described here: http://www.askthebuilder.com/B38_Plaster_Repair_-_Cracks.shtml (the author suggests drywall compound but as Niall pointed out, it's better to use other compounds).
-M