It is a common problem that once someone receives some extra certifications that they are worth more in the market place. If you want to be ethical about this, you should stay with your employer for one year or offer to pay them back for the training when you leave. New employers will often pick up this expense. I required my employees to sign a contract stating that if they left before the year was up, they would reimburse the company for training. It is a great expense, not only for the classes, but add expenses, extra hours for those taking up your slack while you are in training and now a new curve to bring someone else up to speed. This may very well affect your references for the next several years. Try to be fair.