When meeting potential clients at Wedding Expos, there is one response I get more than any other why they aren't having a DJ:
"We are going to use an iPod"
*Head Desk* Don't get me wrong! Using an iPod for a weddings can be appropriate...but the fact is, there is only one time when using an iPod instead of a DJ at a wedding or corporate event is acceptable:
When you do not want dancing, just background music, use and iPod.
And that is it...If you are having a day wedding or a casual after work event, feel free to bust out the iPod with your favorite mix, but the simple fact is if you want dancing you need a DJ or a Band. An iPod cannot read a crowd, and iPod cannot mix songs, and and iPod cannot control what people choose. A DJ can! Over 1/5 clients who book Komodo Music say the reason they went with a DJ is they went to a iPod Wedding and nobody danced. It may have saved the couple some money, but it certainly didn't save the reception. If you are worried about not hearing your favorite songs, ensure you write your "Must Play" list of songs that have to be played at your wedding. A good DJ will ensure every one of your songs will be featured. At the end of the day, you will have more fun at your wedding reception or event if everyone is up dancing and having a good time. It might not seem that important initially, but trust me when I say music is one of the biggest things that people remember about your function. 
(Photo courtesy of weddingbycolor.com)