RoamingGypsy: 18 months is a long way off. Depending on your situation, it might be worth it (for your mental wellness and/or even your finances) to abort mission. Not sure what the relo plan is like but sometimes re-payment is pro-rated when one leaves before the required time period. Ex: 50% after 12 months.
Also, it's not unheard of for a hiring company to build a candidate's relocation re-payment into an offer package for someone they're really interested in. Things may have changed in this economy though. An HR professional once told me everything is negotiable, and also if you don't ask, you don't get. Best of luck to you!