I'm not sure I believe this; the source is The Sun after all. Are Customs officials really so stupid that they have never heard of the Sonic Screwdriver? And does Matt Smith really "like to have it about my person at all times?"
from Ananova:
New Doctor Who's prop gaffe
New Doctor Who Matt Smith was reportedly stopped by security at Heathrow airport - for packing a sonic screwdriver in his luggage.
The 27-year-old actor was flying to Belfast when a scanner revealed the prop in his hand luggage, reports The Sun.
Security officials, who did not recognise the new Time Lord, pulled him to one side believing the gadget to be a weapon.
A source told The Sun: "Matt's a huge fan of the screwdriver so didn't think anything of carrying it with him on the plane. It's basically just a fake prop and doesn't exactly do much.
"But going through the X-ray scanner it probably looks like a weapon, albeit something you'd see in Star Wars.
"Matt was very polite and explained what the screwdriver was. But the guards just looked puzzled. Fortunately he had some Doctor Who promo material on him so was able to show them who he was.
"Once the penny dropped and they had a proper look at the screwdriver, they let him through."
Matt has admitted becoming attached to the device, saying: "I like to have it about my person at all times, just twirling it around and flicking it."