Actually a serious problem.
Firstly, let me preface this with a qualifier. I hold a Private Pilot's Licence so I've got some idea of the workload involved in flying an aircraft at low-level. The people who fly Police / Medevac / Rescue helicopters have my unending admiration. It's an extremely demanding job, and one I'd love to do.
Regarding laser pens, they're nowhere near as collimated as other lasers. The laser cavity is of the order of millimetres in length instead of, say ~30cm for a HeNe or >1m for a CO2 laser. Since beam collimation (and therefore divergence) is a function of cavity length, this means that the beam from a diode laser diverges far more than your classic metrology / machining laser. A divergence of up to 1mm per metre would seem to be a reasonable guess. This isn't a problem with a pointer pen as they're only *supposed* to be used for distances of up to 10m. Scaling this up to the sort of distances we're looking at in aviation we get a beam width of ~1m at a range of 1km. At any reasonable distance this means that they're acting more like searchlights than pointer pens.
The second important thing to note about lasers is that, due to the monochromatic light, when they're reflected by a rough surface you get a speckle pattern. (I invite all those above with laser pens to aim them at the wall now, spreading the beam by aiming it almost parallel with the wall will illustrate this best). This is highly distracting as the reflection does not appear to be steady but "moves" as your hand shakes the beam.
So we have a bright and highly distracting torch shining into our helicopter.
Anyone who's seen a cockpit picture of any reasonably complex helicopter will know it is full of glass displays and shiny metal switches. I expect you could find a decent pic on Airliners.net if you don't believe me. The point being that you don't need to hit the pilot in the eyes to cause him problems. Simply getting the light into the cockpit will ensure that, far from the darkened cockpit he was in a second ago, he is now sitting in a wonderland of flashing fairy lights.
This would be all well and good, but at night the pilot needs to be able to see his instruments in order to safely fly. A helicopter is not like a fixed-wing aircraft, you cannot just let go of the controls for a second while you fetch the map you stupidly left in the bag behind your seat. It needs constant corrections in order to keep it aloft. My flying books state that the average pilot will get himself into a fatal spin within a minute of flying into cloud without reference to his instruments, in a helicopter that will be on the order of seconds.
Hopefully this explanation will serve to show that a moment's idiocy by an unthinking idiot poses a very real threat to the life of all on board the aircraft, and also to anyone hapless enough to be underneath when it crashes.