Most major races ban the use of electronic devices due to safety concerns. However, the Portland Marathon bills itself as "MP3 Player Friendly" and actually promotes the use of MP3's.
I use my iPod during almost all of my training runs but I have not used it during a race before. I'm undecided whether to plug-in during the marathon.
I do find that an up-tempo song provides a lift when I need it. On the other hand, I'm tempted to go without it so that I can fully take in the marathon experience. I enjoy listening to the support of the crowd, the variety of bands on the course, even the panting of fellow runners as I pass them ;-)
There are supposed to be over 72 entertainment groups on the course so I don't think there will be long stretches of nothingness. Then again, I don't think they'll all provide the energy that I get from listening to my hand-picked song list.
Right now, though, I'm leaning towards going without my iPod.