dr-dom: don't get scared or worried, scroll down this page aways until you come to a thread titled "Lets Educate not Berate" filled with lots of useful information.
Celebrity is a bit dressier than some of the other mainstream cruise lines but don't get all panicky. You will have 2 Formal Nights, 2 Semi-Formal ( i think) and the rest of the nights are "Resort/Country Club Casual" Its easier to tell you what's not acceptable in the main dining rooms during dinner - shorts, t-shirts, sneakers, flip-flops and jeans.
There is another thread under the "Ask CruiseMates Staff" board titled " No need to Fear Formal Nights" check it out.
Actually now is a great time to get some really good deals on prom dressess for the girls, and you. Check out sales in the major department stores, check out Bridal Salons for sales, look into Consignment shops or Thrift stores to pick up an inexpensive blazer for the boys, and don't forget Ebay for some great deals on special occassion dresses.