There's 2 issues here
1) Typo squatting laws -- There's a US law against such practices.
2) Level of 'typo' -- There's a wired or some other online magazine article on this that you can pull up via google search. I don't have the exact url. But from what I remember the tip is:
The closer the typo is to the actual domain, the more effective your conversions. However, this also makes you more susceptible to 1) above.
