restraint of trade
Alister: you're mostly right. In fact, the courts will uphold a restraint of trade clause in an employment contract but only if it is reasonable and provides "no more than adequate" protection for the employer. If the contract is too oppressive in any way, then the whole clause is nuked.
Also, it is not true that only senior employees or people with access to customer databases can be covered by non-compete clauses. Funnily enough, considering the AC's comment, mechanics and hairdressers have both been the subject of test cases on this issue.
But what seems to be consensus is that most anti-compete clauses are so poorly written that they are either valueless or actually counterproductive for the employer. One that says " you can't work for anyone within five miles for the next six months " is much more likely to be enforced than "you may not work for anyone within 300 miles for the next ten years" - but non-lawyers *coughHRPEOPLEcough* will think the second one is stronger.