As the original post was concerning legality, it may help to clarify: As previous replies have demonstrated, it is not illegal to wear it as part of national dress. Police officers in Scotland are well aware of this clause in the legislation as is any 1st year student of law or legal studies for that matter. I am confident that merely wearing a sgian dubh with your kilt will not lead to arrest in Scotland (or the rest of the UK for that matter).

Brandishing a sgian dubh in any reckless manner however, may lead to the police responding to a breach of the peace, and it would hardly matter whether there was a blade displayed or hidden within the sheath or no blade at all if the people around you were alarmed by it.

Just because it is not illegal doesn't mean to say that places that have some kind of general security concerns won't refuse access. They will have the right to refuse service or admission entirely at their own discretion and this would be as good a reason as any.

I wear my sgian dubh to black-tie events, traditional events such as highland games, weddings etc. It wouldn't normally accompany me to a rugby or football match or an away trip with the lads. Hope this was helpful.