I've had a VP shunt for 5 years now, and was told that the strength of magnets used in everyday objects isn't strong enough to interfere with shunts. Only VP shunts with programmable valves would be affected by magnets anyway, and they'd have to be very close to the shunt to affect it all. The size and strength of magnets outside medical/industrial applications though isn't high enough to cause a problem - usually it's only things like MRI scanners and very strong magnetic fields that would affect a shunt (not that it would damage it, but could alter the setting of a programmable valve). Get checked out by your neurosurgical unit though if you're experiencing headaches, as headaches can be a symptom of shunt failure, and they can easily check that your shunt's set at the right level.