In Canada, all you have to have is a clause in your Contract of Employment. Detailing to the employment agreement. Simply stating, what kinkamono said, Cannot work for/start a company operating of the similar nature for a set time period after employment in current company, wiether you have be dismissed or you terminated your own position.
However there are ways around this, you may start something which catures to the same nature of that company, which doesn't have similar operating techniques. Build up with current
technologies, and when time has gone through, techniqually you've built the clientel, you've created the goals and achieved them already. So basically you found the clause to that agreement and worked you way out of it, while in term doing the exact same thing as you.
As to Vovex with the following post,
Vovex Technology said:
Yay finally I answer 6 months later 
I still would not allow it, even if it was off the clock. His clients are potential clients of our business.
You can't limit to what a employee does outside of your company. If he/she were to operate a company of similar stature, then you can limit, if the agreement states they cannot. But you cannot limit to the operation of a sole company or venture of an employee, if operations do not stand in the company as the same or similar operational standards.
I took contract law in my first year of college. However, this is canadian law, so it may be different.
