There is no law or requirement for the common left/front, right/rear braking arrangement and you are free to set your own bike up however you prefer. It would be nice to warn anyone you let ride your bike if you make the change.

The original thinking about front/left braking was, as Murray Missile also prefers, that right handed people (most of the population) were less likely to overpower their front brake if they used their "weak" hand. I agree with xenologer that I prefer using my dominant hand for the front brake but since I'm left handed, the conventional setup suits me fine.