I'd imagine they're worried about a kid swallowing more than one with a gap between doing so - then they might attract each other through the walls of the gut and potentially lead to some nastiness that way.

Yes, it is the magnetism at the North pole that makes life as an Eskimo very dangerous. We only imagined it was the risk from Polar Bears, climate and limited food supplies that was their biggest risk.

Seriously - everything in life is dangerous but how dangerous can a low power magnet in a toy be?

The types of magnets concerned are the ceramic so-called 'super magnets', which have a very strong magnetic field, these have been known to perforate the intestine, when swallowed. One such magnet, may pass through the digestive tract, without harm. The problems start when more than one are swallowed and they are in different parts of the intestine. The attractive force that these magnets apply, can be sufficient to rupture the intestinal walls between them.

I think sia and blacksmith are on the right line. I can't believe it's the power of the magnet, not even the very strong rare earth ones. The MRI scanner subjects patients to tremendous magnetic fields yet is the safest of imaging equipment available. It is able to dislodge and move around the body metal clips used in surgery - hence all the questions asked before having a scan. The following videos illustrate the power of the MRI magnet.

http://www.mindhacks.com/blog/2007/09/the_awesome_power_of.html
http://www.mindfully.org/Technology/2005/MRI-Magnet-Accidents19aug05.htm