Advice

My advice on ipod Weddings:
The ipod may seem like a suitable choice for a wedding but before you make any quick decisions it is worth considering a few things:
Requests - A good DJ will play all of your music choices and be able to take requests on the night
Quality - I am afraid that MP3s are good on headphones while jogging... but the sound quality is comparable to radio quality when played through a good PA system. A good DJ will be playing Linear PCM Audio Wave files through a 24bit Dual Stereo soundcard.
Continuity - A disc jockey is there to group together similar styles of music and beat match them so that the dancers keep bouncing to the same rhythm. He will also be able to react to changes in mood of the audience and know what style of music will change the dynamic.
Rapport - It is not just about the music. The rapport of a good DJ as he shares the experiences of your night will give that extra dimension to the event.
Lights - A good DJ will have a suitable LED lighting rig. This will add immensely to the atmosphere of the night.
Less stress - With a DJ at least you will be able to relax without worrying about the need to change the music if no-one is dancing.

My advice on having a DJ instead of a Band
A DJ has a passion for all styles of music. He will be able to play all your music request, current chart material, take requests, read the crowd and adjust the music to suit, cater for multicultural music and play the original recording of your First Dance. He is more likely to have a dedicated lighting rig and be able to set-up earlier and play on longer. He will also adapt to the size of the space available. He will also be the more cost effective option which is helpful if you have a set budget.

My advice on advance song request from guests
At one time it was very useful to have a list of songs in-advance - just in case any special CDs had to be purchased but recently DJs have access to legal Wav file download sites and can buy additional tracks on the night via a 3G connection. I find it useful to have an advance play-list from guests if they are predominantly from another country. I say this because I have needed to import CDs from all over the world for multicultural weddings.
Personally, I recommend that the Bride & Groom just give me their "must play" song list in-advance as I will give the guests cards and pens on the night for their requests. The main reason for this is so I can put a face to the person who asks for a particular song. This is useful for knowing when to play a track. If I notice someone has gone off for some fresh-air or to change their shoes etc... I can hold the track. It is also useful to know which requests have come from families with kids as their songs generally need to be played earlier.
I suppose another good reason for requests to be taken on the night is because it is easier for the DJ to knock-back a song that is on your "do not play" list rather than you having to tell the guest yourself. Also, if any dodgy requests are given to the DJ he can check with you on the night before playing them.
If you are including a song request-slip on your invite can I suggest it says something like "the song I definitely want to dance too is:" as there lots of great songs but not all are ideal for the dancefloor.