US law often fails, but its basic intent is to codify ethical behavior. And ethical behavior should be what you strive for, regardless of the legal requirements where you live.
Think of the following cases and think about where you stand on the ethics involved.
1. You take outside work that directly competes with your employer, and you use training provided by the employer and equipment (computers, software, TE) provided by the employer to perform the work.
2. You take outside work that directly competes with your employer, using contacts made while performing work for the employer.
3. You take work in a market that is not currently served by your employer. Your presence in that market effectively prevents the employer from expanding in that area.
4. You take work in a field that requires knowledge of engineering, but is totally unrelated to your employers business area (High end audio design for outside work for example while the employer does digital controllers for high volume markets).
5. You take outside work that consumes so much of your creative energy that you can barely meet the minimum requirements of your day job.
The details are always fuzzy on these things. The three criteria that jpanhalt measures seem specific, but can easily be muddied. Consideration at the time it is signed (did you get a signing bonus, and was allocation of salary discussed or implied). Legitimate business interests is equally fuzzy. In a large multi-national company is there any facet of commerce that is not of interest to some part of the company. And reasonable in scope, geography and time is a rubbery concept.
The best approach is to discuss it with your employer and come to an amicable agreement. In the real world the employer may not be reasonable, and local law may prevent something that would be acceptable to both you and your employer. But it is best to find these things out before you do something than after.
As a side note. The Radio Shack approach is not rare in the US. But business majors reading this thread should think about how those practices were part of an overall management strategy that resulted in a complete business implosion.