Verbal job offer...now waiting on formal letter?

So I was interviewed twice for a position this past week, and at my second interview they told me that they were offering me the job. I verbally accepted, and we discussed salary, start date, etc. I'm supposed to start the job in a week. The night after the interview (Thursday night), I e-mailed the two supervisors that I interviewed with, and thanked them for giving me the oportunity to work with them and I also said I was excited about working at their company. My immediate supervisor e-mailed me back a few hours later and said he was excited as well, that I will be a huge asset to the team and that he would e-mail me the offer details the next day (Friday) by noon. He told me congratulations and that I would receive my formal offer letter from the company by Monday at the latest. Well, it is now 2 pm on Monday, and I have heard nothing from my supervisor with offer details, nor have I received the formal offer letter. Now I'm freaking out because I'm scared they have changed their minds or something. There is no reason for them to, my background is completely clean (not even a speeding ticket), and they told me in the interview that I was the number one candidate (they are filling two positions, and have interviewed three candidates, myself included). Has this happened to anyone else? It's just that I'm supposed to begin in a week and this week I will be flying to Atlanta to get the rest of my stuff to make the permanent move (the job is in NYC). Should I contact my immediate supervisor today and ask if something is wrong? Any help is appreciated since I'm freaking out right now!!!!! Thanks in advance!