@Paul 98
We had a lecture from the nice man from the police and we asked how could we prove that random numbers weren't an encrypted file? His response was that basically it would be ok genuine researchers at a university to have random numbers.
So if you aren't one of Ross Anderson's students then it would be a good idea to delete any digital camera pictures where you left the lens cap on.
Except that having deleted but recoverable pictures on your hard drive isn't a defence if you are a knowledgeable person (ie. know how to undelete them)
You could scrub the drive, but having a pattern of ranom bits written to the unused blocks would also be suspicous.
Probably safest not to have a computer or a camera these days if you live in the UK.