Carbon offsetting is a very short term method of reducing emissions. Yes, it reduces our emissions this year, and maybe next year, but over the next 20 years it won't make much difference. There is a very limited amount of emissions that can be easily reduced - once people have put enough money into offsetting schemes to reduce all of them the cost of offsetting will grow enormously.
Incidentally - does anyone know where I can find statistics to see if the "A cleaner environment starts with yourself" concept is true? What percentage of carbon emissions come from things a typical person can reduce on their own?