What Car? Q&A: July 2009 - Tyre wear problems
A) Tyre wear isn't just down to the way a car is driven. It's also related to the car's steering and suspension geometry settings. Irregular tyre wear is a common sign that tracking settings are incorrect.
It sounds as though one of your front tyres needs replacing right away, so when you visit your local tyre fitter ask them to check the wheel's settings. This should cure the problem.
Under-inflation can also cause the outside edge of a tyre to wear quickly, so check your tyre pressures on a weekly basis.