CompUSA may be your best bet. I'm have yet to really find any other Apple reseller other than them to offer 0% interest for a time longer than 90 days. Best Buy and Circuit City are usually the retailers to offer good 0% interest for a long time, sometimes 2 years, but unfortuntatly they really don't sell Mac computers. All other Apple dealers usually have financing deals with a high interest rate. I ended up just putting my Powerbook on my credit card that had much lower interest than all the other retailers, and plus so I could get it online for the educational discount.
So my advice, if your anxiously awaiting a Powerbook in your hands, go for the CompUSA deal or if possible use your credit card, but CompUSA might be the wisest choice if you are certain you pay it off in a 18 months. And word of advice, if you go with CompUSA don't let them talk you into their warranty cause they will hound you to get it. Wait a year then purchase AppleCare, because with AppleCare you still can take it into a CompUSA to get it fixed since they are Apple Authorized.