Dictators have used the notion of "secret police" to prosecute their political foes for a long time (certainly, prevalently throughout the 20th century - whether we are talking about Stalin, Hitler or Mugabe). It's fairly clear that the Conservatives would use this office at first to go on merry little witch hunts against the Liberals:
" The independent director of public prosecutions not a politician will decide on prosecutions arising from the sponsorship scandal we will let him do his job without interference"
Hmmm - a little chilling, don't you think? Along with his "we'll tell you you're happy" comment after pushing his "non-confidence" motion through, what's next? If you're not happy, two big guys in trench coats are going to pay you a visit? (Once they find a way to expand the "Office of Public Prosecution"'s powers to include sedition or some such)
I just found an excellent analysis of the legal problems with Harper's proposal.