I just called HMRC about this (13th April 2013), to at least tie down capital gains vs income tax

The first advisor had not heard of it but passed me to someone who had

The advice was very clear. For a UK taxpayer buying and selling bitcoins :

"buying and selling Bitcoins using your own money definitely comes under capital gains tax"
HMRC telephone advisor 13th April 2013

The advisor explained that it is treated like buying and selling a foreign currency.  

I asked if it would ever come under income tax and the answer was 'only if you were using other people's money to buy and sell, for instance running an investment scheme' - (not that pirate at 40 cares I imagine:-)

We didn't discuss mining income, or VAT for businesses.

Thought this might be useful for others. Of course I can't guarantee you'll get the same answer from a different advisor at a later date but this was at least a very clear statement of HMRCs current policy.