I used my MP3 player hooked up to laptop computer speakers at Commader's Terrace, which would be the equivalent to an Ipod speaker. It worked perfectly. DH was worried that the music wouldn't be loud or clear enough, but it was great. According to my WC this is becoming more and more common for those people who want to have a song of "CD quality" and not played by the violinist.
The one thing you need to think of though, is that some of the wedding coordinators don't feel comfortable operating them or setting them up. I told mine that if she had ANY problems to go to my brother, and he'd set it up.
It all worked out splendidly!