5yr arm VS. 30yr fixed

I'm in the process of purchasing a coop on the UWS and I'm watching the 30yr fixed rates go up and up as I'm waiting to sign the contract. I was pre-approved at 4.25 when I first started my search last fall and I'm now getting a rate of 4.875. While this is still very low in the history of interest rates it's still quite a difference in my monthly payments. Now I'm considering a 5yr arm at 3.625. I will save about $400 a month. If I really believed that the rates would still be under 6 in 5 years I would go for the arm. What has me worried is I'm hearing a lot of talk about the rates easily going up over 7 in 5 yrs. I don't plan on living in the apt for a very long time, but 5 years can go by very quickly. Any thoughts?