If its something as minor as a caution, and so minor that it did not even go to court, then it'll probably be deleted by 2009/2010.
I'm also hoping to qualify in the legal profession in the future. I have also found myself 'foul of the law'. I was arrested at an anti-war protest about a year ago under the suspcious guise of the Public Highways Act. However, as that happened before I was 18, it gets deleted when I turn 18. Plus it was only a local police record.
Something I should probably be more worried about is MI5/MI6 (whichever is domestic security?) files if I ever wish to join the Civil Service. Considering I've been on practically every anti-war, anti-Israeli occupation, Solidarity, etc protest in Manchester in the last two years, plus photos of mine were used as evidence of over-zealous policing in some indymedia.org report, and I'm a member of a far-left political group - a file is almost guaranteed. Yay!